Add scalafmt code formatter

I would like to propose adding scalafmt to this codebase for the same set of obvious reasons why having a code formatter is a good idea. The typical problem with adding a code formatter is it causes merge conflicts for currently open PR's however scalafmt is in a stage where there are only a couple of major PR's which are planned to be merged soon i.e.

• https://github.com/sbt/sbt-github-actions/pull/65
• https://github.com/sbt/sbt-github-actions/pull/136
• https://github.com/sbt/sbt-github-actions/pull/66
• https://github.com/sbt/sbt-github-actions/pull/98

The idea would be to introduce scalafmt after these PR's are merged. In regards to the actual scalafmt configuration, I am personally not that fussed. From introducing scalafmt to other projects usually there are 2 approaches to this, either try to find configuration that matches current style as much as possible (which can be a bit of work) or to just use something basic and deal with a large diff

@eed3si9n @armanbilge Thoughts?

Review the four referenced pull requests (#65, #136, #66, and #98) and the current Scala project configuration before choosing an introduction point. Decide whether the scalafmt configuration should match the existing style or use a basic configuration; done means scalafmt is added after the pending changes and the codebase has a consistent formatter setup.