Add support for Scala3doc

[adpi2]

Part of #6080

[adpi2]

This PR depends on:

• Add scala3doc artifact librarymanagement#359
• Add ScalaInstance.loaderCompilerOnly zinc#960

The scala3doc dependency is added only if externalResolvers contains the JCenter repository.

It is added to a separate ivy configuration, called ScalaDocTool, so that we have two distinct lists of jars: one list for scala3-compiler and another one for scala3doc. Then we can easily create the compiler class loader and the scala3doc class loader whose parent is the compiler class loader.

To make doc works with Scala 3, the user must add the JCenter repository manually:

resolvers += Resolver.JCenterRepostiory

Once scala3doc has been migrated away from dokka, JCenter won't be needed anymore and the scala3doc dependency will be added by default.

In order to merge this PR, we need to merge sbt/librarymanagement#359 and sbt/zinc#960 and to release them. Also I will remove the commit 5dfb304 (it forces the CI to run on my zinc and libmanagement branches)

[eed3si9n]

Zinc 1.5.0-M2 and lm 1.5.0-M3 are available.

[adpi2]

Thanks @eed3si9n
It's now ready for review.