Create an abstraction for Completions

[joshuadavidthomas]

Right now completions are interwoven in the document Store and TextDocument, but if we want to support more parts of Django besides template tags, it probably makes sense to abstract it to its own module. I don't have any concrete ideas about the best way to achieve this - my first thought is a Trait that the different parts can implement, but I'm not sure in practice if that makes sense.

[joshuadavidthomas]

Let's get the core finished first before we start thinking about abstractions 🙄