Add Scastie integration

[felher]

It should be able to integrate s3te into scastie, so that one can just open scastie and browse the scala code there without having to install anything.

[felher]

Okay, because of scalacenter/scastie#1054, it seems that scala 3 libraries don't currently work on scalajs. So this will be a larger-than-expected refactoring, splitting the code into different modules and dropping scala3 features from the modules that need to work in scastie.

[felher]

After rewriting to use only scala 2 features and using scalajs-dom with for3Use2_13, there still seem to be some problems, currently tracked by scalacenter/scastie#1102