Insert imports code action

[pepeiborra]

Initial stab at a code action to insert imports based on GHC suggestions. Outstanding issues (to my knowledge):

• the suggestions for missing constructors lead to compile errors when applied, see below. To deal with this, a second code action is provided to fix these imports.

    In module ‘Data.Maybe’:
      ‘Just’ is a data constructor of ‘Maybe’
    To import it use
      import Data.Maybe( Maybe( Just ) )
    or
      import Data.Maybe( Maybe(..) )

• If a missing constructor has the same name as the defining data type (e.g. Identity), neither code action can deal with it properly.

I'd say that this PR gets ghcide 80% there. I will add tests for these outstanding issues later today.

[pepeiborra]

This is ready for review

[cocreature]

This looks great, thanks a lot!