Add script to update inner Scala versions

[valencik]

An attempt at resolving #113

The approach looks like this:

1. We have a script that extracts the Scala 2 and 3 versions from build.sbt using regex (mild 😱)
2. The script then replaces the scala version template values in the inner build's default.properties file
3. We trigger this script anytime there is an update to scala-library or scala3-library_3

Step 1 sounds maybe scary, but I don't think it's that bad. And besides, we review these PRs and have tests anyways.
Step 2 feels normal (although sed on my macbook and sed in CI runner, might be different, will see soon enough...)
Step 3 works because we have this phantomDependencies sbt project in the outer build with the Scala library as dependencies, and Scala Steward correctly updates those.

[valencik]

I have faith this will work, and if it doesn't, I'll fix it.
🚢