feature: ExplicitResultTypes for Scala 3

[tgodzik]

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There is still plenty to do, and I am not 100% sure about the approach, but it's a start. Tests are passing but some outputs are different the Scala 2 ones. I also found some issues in some of the tests, I removed them until I can fix them in the compiler.

TODO:

• [x] extract common parts for both Scala 2 and 3 to a trait/abstract class
• [x] ~fix issues in the compiler~ (non-blocking?)
• [x] download specific compiler for a specific version instead of explicitly setting dependency
• [x] ~separate tests for LTS and Next~ with both on the latest it seem ok, but we might need some compat at some point
• [ ] make sure that each patch is sequentially one after another (currently all are done one the same code, which might cause issues)