Official support for writing community rules in Scala 3

[bjaglin]

Spin-off from #1680 / #2034

• Publish core_3 without using it internally
  • Wait for Scala 3 publishing plan scalameta/scalameta#2485
  • Consider our options for implicit helpers exposed via package not available with -Xsource:3 and Scala 3 #1949
  • Find a way to test community rules built against core3 (custom cli 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
• Backward compatible reflect3
  • 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)
  • Publish cli_3 against core_3 & reflect_3 in a minor (breaking-signalling) release