Add Dotty cross-build

[Philippus]

As mentioned in #221 (comment), it would be neat to add cross-compilation to Dotty to this library.
To figure out how to do this, it might help to look at scala/scala-xml#312, where this is done for the scala-xml library.

[SethTisue]

anyone watching this repo feel like attempting this...? I wouldn't expect the effort to hit any serious snags, it's just vanilla Scala code with no dependencies besides JUnit

[SethTisue]

“good first issue” for someone wanting to dip their toes in the Dotty waters

[SethTisue]

@Philippus should we close this now?

[Philippus]

Closed in #270