Official support for writing community rules in Scala 3

[bjaglin]

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Spin-off from https://github.com/scalacenter/scalafix/issues/1680 / https://github.com/scalacenter/scalafix/pull/2034

• Publish core_3 without using it internally
  • [ ] Wait for scalameta
    • [x] https://github.com/scalameta/scalameta/issues/2485
      • [x] https://github.com/scalameta/scalameta/pull/3939
      • [x] https://github.com/scalameta/scalameta/pull/3347
    • [ ] https://github.com/scalameta/scalameta/issues/4146
    • [ ] https://github.com/scalameta/scalameta/issues/4145 ?
  • [ ] Consider our options for https://github.com/scalacenter/scalafix/issues/1949
  • [ ] Find a way to test community rules built against core3 (custom cli/testkit artifact?)
  • [ ] Publish core with Scala LTS as a patch release (or a minor release in RC if nothing was found for the point above)
• Prepare community rules to have artifacts built against core_3 ready ahead of the last step
  • [ ] Go through the listed community rules and make sure Scala Steward rolls out the change
  • [ ] Advocate for community rules to cross-compile to scala 3 by updating https://github.com/scalacenter/scalafix.g8 & examples in the docs (use scala center twitter account?)
  • [ ] Instead of testing Scala 3 targets with Scala 2.12, test Scala 2.13 targets with Scala 3 in https://github.com/scalacenter/scalafix.g8 since that's what cli3 will take by default
  • [ ] Update testkit scripted accordingly
• Backward compatible reflect3 (source rules written against 2.13 applied by scala-steward on scala 3 projects should continue working)
  • If compilation with dotty fails, try compilation with 2.13 compiler against a core3 overlaid with the compat-metaconfig-macro containing Scala2 macros
• Publish a pure Scala 3 cli_3
  • [ ] Make error actionnable when _3 artifacts are not found for community rules (fallback will no longer works as core_3 is not binary compatible with core_2.13)
  • [ ] Remove advertised usage of for3Use2_13 in local rules docs https://github.com/scalacenter/scalafix/pull/2221
  • [ ] Publish cli_3 against core_3 & reflect_3 in a minor (breaking-signalling) release