Add sbt 2 crossbuilds

[BillyAutrey]

• Bump sbt to 1.10.2
• Add compatibility shim for Scala 2.12/3 crossbuilds
• Scala3 syntax warnings
• Add compatibility shim for to[Set] vs to(Set) to resolve Scala 3 compile errors
• Other minor syntax changes for Scala 3
• Switch imports around and explicitly use a few classes to resolve compile issues in Scala 3

Still todo:

• File/HashedVirtualFileRef conversions for a few keys
• AttributeKey[String]/StringAttributeKey conversions
• Add scala3 to ci

Towards #255

[BillyAutrey]

Closing this to break into multiple PRs.