Custom webview to render sketch + overlays

Instead of using the Live Preview view to render all animations and overlays on the client web page, we can have a custom webview which will render the sketch (from the user's workspace) in a iframe (which will still be served up by live-server), and our overlays (animations, captions, etc.)

This has the following benefits:
- User does not have to copy all client files every time and can start with a blank p5 boilerplate workspace
- More control on overlays
- Prevents refreshing of entire view for live reload (only the iframe containing the sketch will be reloaded)