feat!: ship the attributes in a public library instead of generating them into each project

[glennawatson]

Summary

The attributes are public types in the package's own library, so the generators no longer declare them in every consuming project; task commands can also run in the background, and [BindableDerivedList] output no longer needs DynamicData.

• [Reactive], [ReactiveCommand] and the other attributes, with the AccessModifier, PropertyAccessModifier and InheritanceModifier enums, ship in ReactiveUI.SourceGenerators.dll. It targets net8.0-net11.0 and net462-net481, keeps the existing namespaces, and is referenced by consumers from the package's lib/ folder.
• The attributes are [Conditional], so a consuming assembly keeps no reference to the library. The generators read them from source; defining REACTIVEUI_SOURCEGENERATORS_KEEP_ATTRIBUTES keeps them in metadata.
• The generators no longer register post-initialization output. Nothing is declared into a consumer's assembly.
• The package bundles the generators and code fixes under analyzers/. The Roslyn 4.8, 4.14 and 5.0 builds are now ReactiveUI.SourceGenerators.Roslyn.dll, and each band carries the code fixes, which previously arrived only through a dependency that excluded its analyzers. Consumers now see RXUISG0016 and RXUISG0020.
• [ReactiveCommand(RunInBackground = true)] starts a task-returning method with Task.Run. The command parameter and CancellationToken are passed through, so the code before the first await no longer runs on the calling thread; previously the option was ignored for tasks.
• [ReactiveCommand(BackgroundScheduler = ...)] chooses the scheduler a synchronous background command runs on. It accepts a scheduler member or a built-in ReactiveUI scheduler, as OutputScheduler does, and implies RunInBackground.
• [BindableDerivedList] output no longer opens with using DynamicData;. The generated property only exposes the field, so a project without DynamicData or System.Reactive can use it.

Why

Projects that share internals through InternalsVisibleTo failed with CS0436 when both used the generators, and the attribute namespace only existed when the generator ran.

• Every consuming assembly received its own internal ReactiveUI.SourceGenerators.ReactiveAttribute and friends, so an assembly granted access to another's internals saw two copies of each.
• Because the namespace was generated, an IDE that did not run the generator reported ReactiveUI.SourceGenerators as missing; it now comes from a referenced assembly.
• The injected attributes also used init accessors, which do not compile on .NET Framework without an IsExternalInit polyfill.
• Task commands had no way to leave the calling thread without wrapping each method in Task.Run, and the background scheduler could not be chosen.
• [BindableDerivedList] forced a DynamicData reference, and with it System.Reactive, on projects that fill the collection by other means.

Closes #371
Closes #347
Closes #489

Breaking changes

The package is no longer a development dependency, because consumers compile against its attributes.

• A PackageReference with an <IncludeAssets> line that omits compile (older installs added one) must drop that line: <PackageReference Include="ReactiveUI.SourceGenerators" Version="..." PrivateAssets="all" />.
• The attributes' named properties have set accessors instead of init; attribute syntax is unchanged.
• The code-fix analyzers now run in consuming projects, so misplaced [Reactive] attributes report RXUISG0020 as a warning.
• A task-returning method with RunInBackground = true now starts on the thread pool; remove the option to keep it on the calling thread.

How this was verified

The generator tests compile against the attributes library, as a consumer does; new tests run generated task commands to check the thread they start on, and compile [BindableDerivedList] without DynamicData; the packed nupkg was exercised by hand.

• A library granting InternalsVisibleTo to an app, both using [Reactive], reproduces CS0436 on the previous package and builds and runs on this one; a net472 consumer builds. The VS2022 editor behaviour in [Bug]: Namespace ReactiveUI.SourceGenerators doesn't not exists in VS2022 when updated to 2.6.1 #371 was not reproduced here.

Notes for the reviewer

Start with src/ReactiveUI.SourceGenerators/ and its project file, then ReactiveCommandGenerator.Execute.cs; most of the rest is mechanical.

• Hand-written: the attribute sources and ReactiveUI.SourceGenerators.csproj, AttributeDefinitions.cs, the generators' Initialize methods, TestHelper.cs, and the command generator's WriteExecuteArgument and WriteSchedulerArguments.
• Mechanical: the deleted *Attr.verified.cs and *AccMod.verified.cs snapshots of the injected files, the one-line using DynamicData; removal across the DERIVEDLIST snapshots, the project references added to the test, sample and benchmark projects, and the Roslyn projects' AssemblyName.
• Subtle: ReactiveUI 24 has no CreateRunInBackground(execute, backgroundScheduler) overload, so a background scheduler without can-execute or output scheduler is written with canExecute: null, which also binds on ReactiveUI 23.
• BackgroundScheduler does not apply to task-returning methods, which always start on the thread pool, or to observable-returning methods.
• Out of scope: the separate ReactiveUI.SourceGenerators.Analyzers.CodeFixes package is still packable on its own; nothing depends on it any more.

Checklist

• I have read the Contribute guide
• The PR title follows Conventional Commits
• Tests cover this change, or the summary says why they do not
• New or changed public API has XML documentation

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.45%. Comparing base (d2e6f72) to head (f4a0c8b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #503      +/-   ##
==========================================
- Coverage   96.94%   96.45%   -0.50%     
==========================================
  Files          69       71       +2     
  Lines        3857     3578     -279     
  Branches      503      506       +3     
==========================================
- Hits         3739     3451     -288     
- Misses        118      127       +9     

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