add a Scala 3 crossbuild

[SethTisue]

probably it works anyway with withDottyCompat, but:

• adding withDottyCompat is a bit of extra hassle
• even if withDottyCompat appears to work, we don't want users to be wondering whether it really works or just, like, usually works
• it would be good to get an early start on making the code compilable on Scala 3, and perhaps the Scala 3 compiler will find issues the Scala 2 compiler missed
• it would permit adding the repo to the Dotty community build, to increase the chances of catching compiler regressions

and so on.

volunteer?

note that there is almost no code at all in compat/src/main/scala-2.13, nearly all of the code in this repo is for 2.11/2.12 only, so the likelihood of running into any difficulty here is pretty small

once done, I'd be happy to take care of adding it to the Dotty community build.

[SethTisue]

community build PR: scala/scala3#10427