Scala 3 support

[bjaglin]

Support for running rules against Scala 3 sources has just been added to scalafix & sbt-scalafix (SNAPSHOTs only for now, release upcoming next week). Metals will probably follow shortly.

Very few built-in rules are supported for the moment (many depend on features unavailable in the Scala 3 compiler). Adding support for OrganizeImports might not be trivial either (although we might get a good surprise at the first run of the testkit against Scala 3 input sources), but I wanted to open this early to see whether/how we could collaborate on this. I see it as an opportunity for Scalafix maintainers (and potentially Scalameta maintainers) to focus on the right things, and for OrganizeImports to get support faster.

Some pointers:

• Scala 3 support scalacenter/scalafix#1316
• WIP - How to deal with rules that wont work right now in Scala 3 scalacenter/scalafix#1394 (review)

Happy to hear your thoughts!

cc @mlachkar

[bjaglin]

#179 tests and documents the current status of running scalafix against Scala 3 source files.

Known limitations are documented in the README.

[bjaglin]

I guess this can be closed as Scala 3 is now considered supported, so remaining work should be tracked in separate issues.