[LSP] Listen to file system events

[msujew]

In order to properly react to changes happening on the file system level, there are two separate channels:

1. There are events sent by the language client if the user deletes files in the editor UI. See workspace/didDeleteFiles and similar notifications.
2. Language servers can register watchers on the client, which then propagate any non-editor fs changes to the server. See workspace/didChangeWatchedFiles.

[msujew]

Will be much easier to implement once #130 lands.