support for Scala 3

[SethTisue]

a number of our modules have already landed support for Dotty / Scala 3

it would be nice if this plugin had some basic Scala 3 awareness around things like what compiler options are supported

on the other hand, since Scala 3 is still such a rapidly moving target, maybe it's better for now to let each module go its own way? there are downsides to moving things here: it becomes less clear to individual maintainers where things are coming from

in any case, I wanted to open at the ticket. at minimum, maybe we can record useful knowledge here

[SethTisue]

these modules are already on Scala 3:

• scala-collection-compat
• scala-parallel-collections
• scala-parser-combinators
• scala-swing
• scala-xml
• scala-java8-compat

these aren't, but have help-wanted tickets:

• Add Scala 3 crossbuild scala-collection-contrib#119
• add Scala 3 crossbuild scala-library-next#14

not cross-buildable:

• Scala 3 / Dotty plans? scala-async#232

[SethTisue]

I guess let's not leave this open indefinitely. Scala 3 support is working fine in many of the modules. There isn't any specific known need to make any further changes in this repo.