Make vs2019/vs2026 fully-functional platform targets; default to vs2026 when installed

[danoli3]

Summary

• vs2019/vs2026 were essentially inert as PG platform targets: several hardcoded platform == "vs" exact-match checks scattered through the C++ backend (most severely ofAddon::checkCorrectPlatform()) silently dropped an entire addon's Windows-specific addon_config.mk directives for these targets.
• Fixes all the platform-string-exact-match sites (addon config parsing, DLL copy exclusion, template/overlay matching) to treat vs/vs2019/vs2026 as equivalent MSVC/Visual Studio targets where appropriate, while keeping their distinct toolsets (v143/v142/v145) intact.
• GUI: on Windows, auto-detects VS2026 via vswhere.exe and pre-selects it over the vs (2022) default when installed; platform dropdown now explicitly lists Visual Studio 2019/2022/2026 as separate entries (each locks a distinct MSVC toolset, so this is deliberate, not redundant).

Test plan

• Rebuilt the commandLine binary and generated real test projects for vs2019/vs2026/vs targets with an addon that has Windows-specific addon_config.mk directives; confirmed the directives are now applied (previously silently dropped) for vs2019/vs2026.
• Verified vswhere.exe-based VS2026 detection logic against the existing getVisualStudioPath() implementation.