Allow to use watchman as backend for file watching

[bpasero]

This came up in #156690 (comment)

//cc @vilk-stripe

[jvilk-stripe]

If you surface this in an experimental option, let me know and we can try it out for our use case at Stripe. This would make our local development and remote development workflows more efficient.

[bpasero]

A quick test on macOS running with watchman backend shows that not all of our parcel watcher integration tests pass with that backend:

4 failing

  1) File Watcher (parcel)
       subsequent watch updates watchers (path):

      
      + expected - actual


  Error: Timeout of 5000ms exceeded. For async tests and hooks, ensure "done()" is called; if returning a Promise, ensure it resolves.

  2) File Watcher (parcel)
       wrong casing:

      
      + expected - actual


  Error: Timeout of 5000ms exceeded. For async tests and hooks, ensure "done()" is called; if returning a Promise, ensure it resolves.

  3) File Watcher (parcel)
       deleting watched path is handled properly:

      
      + expected - actual


  Error: Timeout of 5000ms exceeded. For async tests and hooks, ensure "done()" is called; if returning a Promise, ensure it resolves.

  4) File Watcher (parcel)
       "after each" hook for "deleting watched path is handled properly":

      
      + expected - actual


  Error: Timeout of 5000ms exceeded. For async tests and hooks, ensure "done()" is called; if returning a Promise, ensure it resolves.

[vscodenpa]

We closed this issue because we don't plan to address it in the foreseeable future. If you disagree and feel that this issue is crucial: we are happy to listen and to reconsider.

If you wonder what we are up to, please see our roadmap and issue reporting guidelines.

Thanks for your understanding, and happy coding!