POC: support sbt 2

[kubukoz]

No description provided.

[kubukoz]

sbt '+3; sbt-tpolecat-plugin/compile hits a compiler crash: https://gist.github.com/kubukoz/900ebaee4eeb07a793b71be3cb13b903

[error] ## Exception when compiling 2 sources to /Users/kubukoz/projects/sbt-tpolecat/plugin/target/scala-3.3.4/sbt-2.0.0-M2/classes
[error] dotty.tools.dotc.core.TypeError$$anon$1: Toplevel definition overrideConfigs is defined in
[error]   /Users/kubukoz/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/scala-sbt/main_3/2.0.0-M2/main_3-2.0.0-M2.jar(sbt/BuildCommon.class)
[error] and also in
[error]   /Users/kubukoz/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/scala-sbt/main_3/2.0.0-M2/main_3-2.0.0-M2.jar(sbt/ProjectExtra.class)
[error] One of these files should be removed from the classpath.

[kubukoz]

also, help welcome on making the CI build actually do the cross-version thing. I'm not sure how to handle the situation with a varying set of Scala versions (the scalafix project probably doesn't need 2.x yet)