Refactor iframe communication and use MessageChannel

[riknoll]

This PR does a few things:

1. Refactors the IFrameDriver into a base class and adds a new EditorDriver that takes the place of the old class
2. Adds another IFrameDriver that can be used to drive the asset editor embed (arcade.makecode.com/--asseteditor)
3. Adds a new IFrameEmbeddedClient that can be used within the embedded iframe to handle communication with the parent frame
4. Refactors our messaging protocol to use a MessageChannel so that we can support multiple iframe embeds at the same time

This is all 100% backwards compatible with old embeds. To use the new MessageChannel stuff, you need to add ?frameid=<some-unique-id> to the iframe's src url, then everything will just work and it will automatically switch to using the MessageChannel. If you don't add that, the new code falls back to using the good old postMessage for communication. I went ahead and added it to the embed for the teacher tool and everything is working fine.

We still have a few iframe embeds that I'll want to eventually switch over to this new stuff:

• pxtrunner (both --docs and --run)
• the simulator (this will involve refactoring the existing pxsim SimDriver class)

[eanders-ms]

Looks good. (Lint disagrees, but I'm not sure why).