Add Scala 3 artifacts and binary versioning

[adpi2]

Part of sbt/sbt#6080

Implement the binary version logic for Scala 3:

scalaVersion	scalaBinaryVersion
3.0.0-M2	3.0.0-M2
3.0.0-M3-bin-SNAPSHOT	3.0.0-M3
3.0.0-M3-bin-20201215-cbe50b3-NIGHTLY	3.0.0-M3
3.0.0-RC1	3.0.0-RC1
3.0.0	3

3 will most probably become the official binary version of the 3.x series where x < T (scala/scala3#10244 (comment)).
It is already implemented here so that sbt 1.5.0-M1 that will precede Scala 3.0.0 release does not conflict with it in terms of binary version.

Add the Scala 3 artifacts

• scala3-compiler
• scala3-library

[adpi2]

I've added the following tests (validated by @sjrd):

scalaVersion	scalaBinaryVersion
3.1.0-M1	3
3.1.0-RC1-bin-SNAPSHOT	3
3.1.0-RC1	3
3.1.0	3

[adpi2]

sbt/sbt#6207 should be merged before or just after this one to fix the compilation of sbt with this librarymanagement version