CI: Add formatting checks.

[PapyChacal]

Add an sbt plugin for scalafmt, enabling to run sbt scalaFmtAll to autoformat the whole project.
Add a root .scalafmt.conf for (default at this point) format configuration.
Add a CI check that all code respect this format - we can configure it progressively If we dislike anything.

Intent:

• decouples any formatting discussion from any feature/code addition
• Get consistent and at least relatively clear format, always, with no (format-related... 😉) headaches

I'm fine waiting to merge this if this is annoying to anyone. VSCode has nice support for this, where you can autoformat using the config on save, happy to document if this helps anyone

[baymaks]

I like it!