Experimental annotation behaviour redesign

We need to rethink how @experimental should work to:

• fix the problems with bootstrapping the compiler (until then 3.0.0 has to be used as the reference compiler)
• eliminate strange behaviours and unexpected limitations (e.g. https://github.com/lampepfl/dotty/issues/13091)
• take into account the expectations of the community regarding this feature

Related discussion: https://contributors.scala-lang.org/t/experimental-annotation-behaviour-redesign/5187

[smarter]

LGTM (I made a few small edits)

The RHS of an experimental def, val, var, given or type are experimental scopes.

This should be “is an experimental scope” or the beginning of the sentence should be changed (same in a few other rules)

Question: Should we disallow @experimental def main(…)?

I think it’s not too big a deal to leave that escape hatch in since at the point where you’re adding@experimental everywhere in your program it should be obvious you’re doing something weird 😃

[nicolasstucki]

I think rule 1 should also mention given

Updated and added other missing definitions