Start Mix for formatter plugins

[JesseHerrick]

Problem

In #77, @ogomezba reported that our formatter server doesn't work with Quokka. The reason for this is that Quokka attempts to call Mix functions that rely on the Mix supervisor tree being started.

Solution

• Start Mix's supervisor tree in the persistent BEAM formatter server so plugins can call Mix APIs like Mix.Project.config/0. This only starts Mix's tree, so we don't run the risk of a performance hit from trying to start the app itself.
• Add a regression fixture for formatter plugins that require Mix.Project.config/0, matching the Quokka failure mode.

Fixes #77