Update browser extension to use new inlined code intel APIs

[philipp-spiess]

We need to change the browser extension so that the enableLegacyExtensions check runs before we initialize the extensions controller and branch off and use the noop controller if it is false. This way we avoid having to load any of old code intel extensions (not even the bundled ones) and can make a flat bundle.

This might need some special consideration since some work might now be moved to the main thread that was previously done in the extension thread.

/cc @muratsu @jjinnii @ryankscott