Automatically add missing imports

[kansson]

Make it possible to add missing imports automatically when accepting autocomplete suggestions.

[asharkhan3101]

This feature however requires that we do full parsing of workspace in the background without user's knowledge which is not something I am planning to do for now so I am deferring it for now. It's not that we don't do workspace parsing, In order to support references and rename LSP capabilities, we do full workspace parsing but this is controlled by user and lazily done (i.e only when invoked).