This is meant to be minimal for now and is expected to change as the
config system evolves.
Features like word completion should be able to hook into this to
initialize or clear the word index when the toggle for the feature is
turned on or off (respectively).
This adds events for:
* a document being opened
* a document being closed
* a language server sending the initialized notification
* a language server exiting
and also moves some handling done for these scenarios into hooks,
generally moving more into helix-view. A hook is also added on
`DocumentDidChange` which sends the `text_document_did_change`
notification - this resolves a TODO in `document`.