Implement Picture-in-Picture support

[christriants]

This PR implements Picture-in-Picture support using the Document Picture-in-Picture API, and extension of the Picture-in-Picture API.

While this is an experimental technology, and browser-support is limited for now, I thought this would be cool to try out. Using the Picture-in-Picture API seems less optimal because it's only supported on the video element, but we are currently drawing our video to a canvas for playback. The Document Picture-in-Picture API makes it possible to open an always-on-top window that can be populated with arbitrary HTML content — for example a video with custom controls or a set of streams showing the participants of a video conference call.

The core logic to support this function exists in the Picture-in-Picture Button component. togglePictureInPicture handles calling documentPictureInPicture.requestWindow, which opens the Picture-in-Picture window for the current main browsing context. We then take our canvas and append it to the picture-in-picture window, and update its width/height to 100% to fit the window. We then add an event listener for pagehide, which handles re-appending the canvas to the player container when the PiP window is closed.

Other additions:

• We added a store.ts file, which could potentially be used to share state in multiple places. Right now, we're sharing pipState between watch.tsx and PictureInPictureButton.tsx
• We created a global.d.ts file, so we could add declare a global type for DocumentPictureInPicture
• We added a slate in watch.tsx to display in place of the canvas when the video is in PiP mode

Follow-up items:

• Add player controls to the PiP window

[christriants]

Fixed merge conflicts.

I haven't moved controls to the picture-in-picture window yet, nor have I added play/pause/mute functionality to it. Let me know if it's OK to file these as issues, rather than tackle those here!

[christriants]

I haven't moved controls to the picture-in-picture window yet, nor have I added play/pause/mute functionality to it. Let me know if it's OK to file these as issues, rather than tackle those here!

Added basic functionality to play/pause the video from the PiP window by clicking on the window itself. Ideally, we wouldn't have to pass in a prop to this component, and instead, we could just access player.play from state. Even better, we could just render our existing PlayButton in the window, which would be easier / cleaner without prop-drilling.