Cross-compile for 2.13

[kubukoz]

This is a nice-to-have, but I think the migration (and cross-compilation of downstream libraries) to Scala 3 can be made easier if there's a single syntax usable from both 2.13 and 3.x.

Is this planned? Related: #52, #53, #54

[kubukoz]

actuallyyyyyy I realized Scala 3 still supports XML literals, so the cross-compilation should be possible even today. Never mind.

Looking forward to using this though!