Remove our implementation of the scripting api and use upstream's
version. It was recently moved to `extensions/` by
https://chromium-review.googlesource.com/c/chromium/src/+/7784831,
so we link it directly.
Update `ElectronExtensionsBrowserClient` to overrides `IsValidTabId()`
and `GetScriptExecutorForTab()` to provide tab validation and
script-executor hooks.
Remove now-redundant local copy of `scripting.idl`.
Upstream now provides everything we used this for.
Updated breaking-changes.md to document a CSS matching difference.
Co-authored-by: GitHub Copilot <github-copilot[bot]@users.noreply.github.com>