Rewrite watcher

[novemberborn]

This PR aims to rewrite the watcher implementation.

Goals:

• Remove chokidar dependency, rely on fs.watch().
• This will require some additional logic for Linux (where we cannot watch recursively), but we don't need the full behavior that chokidar (or similar) provides
• Simplify logic
• Use static analysis for dependency tracking (https://github.com/vercel/nft)
• Use self-hosted integration-style tests

And as a follow-up goal, integrate with @ava/typescript so changes to say src/ are detected, and depending on whether it assumes a running tsc wait for build/ to be updated, or run immediately since it compiles prior to testing. But this is beyond AVA's current watch mode so can come later.

[novemberborn]

Closing in favor of #3218.