default to client watcher pattern ala LSP

We have a nice custom watcher built into our server using `fb-watchman`, however vscode and other LSP oriented IDE clients would expect to be able to provide their own, and for this server to respond to workspace watch/file change notifications sent from the IDE client.

Thus, we need to decouple this watcher from the server such that it doesn't bring many additional dependencies, and the `fb-watchman` dependency itself would become an `optionalDependencies` entry that we can suggest to the user if they are working with an IDE that expects a server watcher, and they aren't using an LSP client compatibility layer with a built-in watcher for their IDE (which is probably common). Using a server side watcher is possible, but is against the reccomendation of [the microsoft LSP docs](https://microsoft.github.io/language-server-protocol/specifications/specification-3-14/#workspace_didChangeWatchedFiles).

We could also ship it as an additional wrapper package, of the server bundled with fb-watchman. no harm in adding more packages to the monorepo. It could also move to a contrib space