Make watch mode wait for the file system to be idle, and have that be configurable

Based on discussion in https://github.com/avajs/ava/discussions/2702, projects with more complicated build setups may want to configure how long watch mode waits for the filesystem to become idle, lest it runs tests prematurely.

~We currently wait at least 100ms, but debounce for another 10ms after changes. ~Instead let's introduce a watchFsIdle option that defaults to 500ms. This should be configurable through config files only, not CLI flags. We should take time strings, like the timeout option.

Other name suggestions welcome.

The watcher should wait the duration of idle time after the last change. If another change is detected, it should again wait the duration of the idle time.

This likely impacts the current watcher tests which are rather brittle. We also have some integration tests that may be affected.