Setup mima to check for backwards- and forward- incompatible change

[smarter]

We need to add https://github.com/lightbend/mima/ to our build and run its checks as part of our CI, disallowing both backward-incompatible and forward-incompatible change (that is, mimaCheckDirection needs to be set to both), eventually we'll also need to tweak it to allow forward-incompatible changes after a minor version bump.

[smarter]

To be clear, mima should only be enabled in scala3-library, scala3-library-js, scala3-interfaces and tasty-core, for everything else we make no guarantee (and I guess we could also document this clearly by setting versionScheme := Some("semver-spec") in these projects if I'm not mistaken /cc @sjrd)

[sjrd]

Well, after more time than I wish to admit, I have finally discovered that MiMa simply does not support Scala 3 artifacts at the moment. :( The fix is in main, but not released.

I have requested a release at lightbend-labs/mima#619