Pomelo: graphical forth shell

[spookyvision]

This PR adds a graphical forth shell to pomelo, simulating the experience of an actual device. The previous terminal shell is available in parallel.

[hawkw]

i'm fine with merging this as-is --- it's great that it works!

i had some thoughts for potentially improving the way we handle the JS callbacks, though. if you look at the way the drivers for Real Hardware platforms like the D1 work, we tend to have an interrupt handler (which, like a JS callback, can't execute async code) wake the async task that does all the Real Async Work like sending responses to clients of the driver and so on. i wonder if it would be possible to use a similar design here, instead of having to spawn a new task in the callback just to send to a channel.

i understand if you'd prefer to get this PR merged sooner without rewriting that code, though, so i'm fine with saving that for a follow-up. it might help us avoid some of the problems we've had with callback/async interactions, though...