Add support for native ProseMirror widget decorations

[smoores-dev]

Adds a new component type, NativeWidgetView, that renders toDOM-style ProseMirror widget decorations within a wrapper span. Added the gapcursor and tables plugins to the demo for testing (both rely on widgets). This ended up being pleasantly simple, I think!

In testing with the tables plugin, I found and resolved two more issues:

• Fixed a tiny bug where we weren't passing the correct this context to NodeView.update/destroy, which breaks custom node views that are implemented as classes/rely on this context
• Went back to using flushSync rather than batch in dispatchTransaction. I don't remember why we initially switched to batch here, but we need to flush state updates all of the way through to the render function of the useEditor hook on each view.dispatch, so that view.state gets updated in time for the next call to view.dispatch.