Use `watchfiles` instead of `watchdog`

[JoshKarpel]

https://watchfiles.helpmanual.io/ is a more modern take on file-watching and looks like it has a much more convenient interface. It also supports async, which will be helpful for the inevitable textual migration.

Since the code will be changing anyway, we should probably watch the cwd by default, not just the deck file. That should produce more consistent and obvious behavior.