Reenable auto extend imports and drop snippets for infix completions

[pepeiborra]

This builds on the work done by @gdevanla in haskell/ghcide#930 and @berberman in #1246 to reenable completions to automatically extend import lists.

There were two issues that led to this feature getting disabled:

1. the imports were extended multiple times. This is addressed by using a Command that recomputes the edit on invocation, instead reusing the edit computed at completion time
2. the imports were extended wrongly. This is addressed by using ghc-exactprint to ensure the extension is always well formed

While I was there, I also took the chance to drop snippets when typing an infix combinator

[konn]

LGTM 👍