Update diagnostics when editing

[floitsch]

When the user edits the code, a change-notification is sent to the server. While waiting for new diagnostics, the client should update existing diagnostics. In particular, all diagnostics that are after a changed text-entry (removal or insertion) should just be updated. This gives a much smoother editing experience (and is, what vscode does).

[natebosch]

I think this could be possible in very recent versions of vim that have support for text properties but I haven't dug in yet.