Remove polyfills natively supported across the browser support matrix

[Copilot]

Every polyfill in this package except requestIdleCallback and the Invoker Commands API is now natively available in all browsers selected by our Browserslist policy (last 10 Chrome/Edge/Firefox versions, last 3 Safari major versions, Firefox ESR). This drops ~9 modules and one dependency from the shipped bundle.

src/index.ts

The registry retains only the two polyfills still doing work — Safari lacks requestIdleCallback, and Firefox ESR / older Safari majors lack command/commandfor:

import * as requestIdleCallback from './requestidlecallback.js'
import * as commandAndCommandFor from 'invokers-polyfill/fn'

export const polyfills = {
  requestIdleCallback,
  commandAndCommandFor,
}

baseSupport is intentionally unchanged: its checks are independent feature detection for the support floor, not mirrors of the polyfill registry.

Deleted sources and tests

clipboarditem, element-checkvisibility, navigator-clipboard, object-groupby, map-groupby, promise-try, promise-withResolvers, iterator-helpers, set-methods — plus their counterparts in test/.

Dependencies

@oddbird/popover-polyfill removed via npm uninstall (package.json + package-lock.json). invokers-polyfill stays.

Support matrix docs

docs/index.html resolves each row's data-polyfill key against the exported registry, so rows for now-absent keys would render as unsupported. Removed the rows for clipboardItem, elementCheckVisibility, navigatorClipboard, and both popover entries.

Note for reviewers

The beforetoggle on Dialog row references data-polyfill="dialogToggleEvents", which was already missing from the registry before this change. Left as-is to keep the diff scoped — worth a follow-up.