Add Scala 2.13 and Scala 3 projects to scripted tests

[keynmol]

We can have just 1 scripted test which is a multi-module project for 2.12, 2.13, and 3.x projects which should use locally published mdoc plugin.

I'm particularly interested in using this as an integration test for the most common Scala 3 usage, as this is where most issues seem to happen.