Implement screen track capabilities

[Regaddi]

This implements screen sharing capabilities for the local participant.

New APIs (on VoiceClient and Transport):

• startScreenShare(): void
• stopScreenShare(): void
• isSharingScreen(): boolean

New events:

• onScreenTrackStarted(track: MediaStreamTrack, participant: Participant)
• onScreenTrackStopped(track: MediaStreamTrack, participant: Participant)
• onScreenShareError(errorMessage: string)

React updates:

• useVoiceClientMediaTrack now supports screenAudio and screenVideo
• VoiceClientVideo now accepts an optional trackType prop, which can be "video" | "screenVideo" and defaults to "video"

This is minimalistic support without detailed controls over the screen share. We can add configurable options for getDisplayMedia later, if we need to. For now calling startScreenShare() will trigger the default screen share prompt.

[jptaylor]

Holding on this PR until RTVI supports it on the bot side