Implement DidCloseTextDocument LSP handler

[retailcoder]

Specification (LSP)

The document close notification is sent from the client to the server when the document got closed in the client. The document’s master now exists where the document’s Uri points to (e.g. if the document’s Uri is a file Uri the master now exists on disk). As with the open notification the close notification is about managing the document’s content. Receiving a close notification doesn’t mean that the document was open in an editor before. A close notification requires a previous open notification to be sent. Note that a server’s ability to fulfill requests is independent of whether a text document is open or closed.

The server should update its internal document state upon receiving this notification, and write log entries as appropriate.