Local screenshare support

[thyal]

Adds support for starting/stopping local screenshare.
This is done in the room connection hook - not the local media hook.
The local screenshare will be added to the array of screenshares, but it will have a isLocal: true prop, so it's easy to filter on your own screenshare.

Minimal example of usage:

const { actions } = useRoomConnection(...);

const { startScreenshare, stopScreenshare } = actions;

return (
  <button onClick={() => startScreenshare()}>Start Screen Share</button>
  <button onClick={() => stopScreenshare()}>Stop Screen Share</button>
);