Consider a `onVFSChange` callback

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.