New watch mode

[novemberborn]

#3123 rewrites the watch mode. If you frequently test with --watch, I'd appreciate if you could trial that branch:

npm i -D avajs/ava#rewatch

It doesn't yet work on Linux. Behaviorally it should match the existing behavior, but this also does dependency tracking for ESM.

Thanks!

[novemberborn]

Linux might now work. At least the code seems to work when I disable recursive mode on my Mac.