Upgrade clipboard-polyfill to drop custom typings

[aduth]

🛠 Summary of changes

Updates clipboard-polyfill to the latest version to be able to remove a workaround which was added in #5977 for compatibility with default TypeScript DOM types (see upstream lgarron/clipboard-polyfill#148).

Draft until the new version is stable, not release candidate

📜 Testing Plan

As of #7563, clipboard-polyfill is only relevant for tests, so there should be no user-facing impact by this upgrade.

1. yarn test and yarn typecheck should both pass

[aduth]

I may need to put this on pause for now, as it appears that clipboard-polyfill is going ESM-only with the v4 release, and our Mocha tests still rely on CommonJS, which is not compatible. We should probably seek to go ESM, but it's a bit tricky to do for the moment, since we use Babel's on-the-fly transpilation, for which there's not stable support in ESM currently.

See the "Note" at the very end of the above documentation:

Note: @babel/register does not support compiling native Node.js ES modules on the fly, since currently there is no stable API for intercepting ES modules loading.

I notice there are some community projects implementing the experimental Loader API, but (1) I'd prefer something official and (2) as not-unexpected, the experimental APIs have changed such that those projects don't appear to be functional anymore anyways.