This entails not only updating the package, but also:
- Setting the core-js version in the babel config.
- Dropping @babel/polyfill and using core-js directly.
- Removing superfluous polyfills (e.g. URL).
- Removing superfluous runtime module replacements in evergreen.
- Modifying several entry points to use boot/polyfills rather than their
own code, including the server entry point.
- Minor lint fixes.

Remove polyfill-specific lib/url tests.

Don't use boot in standalone notifications.

Don't depend on boot from packages.

Update core-js to 3.6.2 to fix regex bugs.

Switch corejs option in preset-env to `3`.

Update core-js to 3.6.3.

For some reason, it's not included in `stable` as it should be.

Also ensure that gridicons are transpiled.

This helps avoid the globalThis workaround.