This brings things more into line with modern conventions for
build target names, and means we have fewer sources of truth for
the minimum Go version that's needed.
Exposes what I consider a good public API from the `internal/formatter`
package.
Having a go-native formatter is an awesome thing, especially because it
allows shipping the formatter as part of other projects. It's also fair
to keep the formatter's code in `internal`, yet third party projects
should have a way to use relevant parts of it.
