Consider a onVFSChange callback

[michaelpj]

At the moment, if a server wants to react to document changes, it has to add its own handlers for the various document change notifications. This is somewhat repetitious, and anyway lsp is managing these inside the VFS for you.

Contrast with what we do for config changes, where we abstract this away and just offer an onConfigChange callback. This is necessitated because config state changes are not always triggered by a notification, but perhaps the general principle is good.

Hence I think maybe we want to offer some kind of onVFSChange callback which might offer a more straightforward interface for common usecases.