Add a Scalafix migration for v0.5.0

[DavidGregory084]

This uses the ScalafixProjectPlugin that @armanbilge extracted in typelevel-scalafix. Unfortunately we can't use all of the conveniences of sbt-typelevel here yet. 😛

Fixes #86.

[armanbilge]

I was a bit confused at first why the build changed so much :)

[armanbilge]

Btw I haven't forgotten your question from the other PR about considering whether we should fold sbt-tpolecat under sbt-typelevel. There's pros/cons both ways, it won't be an easy one :)

[DavidGregory084]

I was a bit confused at first why the build changed so much :)

I should have converted this to a multi module build a long time ago, but I actually quite like the top-level settings style 😢

[DavidGregory084]

Btw I haven't forgotten your question from the other PR about considering whether we should fold sbt-tpolecat under sbt-typelevel. There's pros/cons both ways, it won't be an easy one :)

Yeah, and I guess it's not a super urgent question - but we may only get this opportunity once as the planets aligned and both projects are working on 0.5.x at the same time 😁