Implement new main annotation

[nicolasstucki]

Followup to #14558 adding the missing new main annotation form #13727.

This is a finished version of the new main annotation design of #13727. This is not necessarily intended to be the final version. We want to add it as experimental to allow users to try it out and promote further design discussions in #14977.

[anatoliykmetyuk]

Hmm, so this annotation does exactly what @main does with the purpose to test out the new implementation and possibly replace @main in the future? I believe having two annotations doing the same thing will generate confusion. We should instead replace @main with this annotaiton right away and fix any regressions as they are reported IMO.

[julienrf]

IMHO, the problem is not the risk of introducing regression, but the risk of introducing new features that will have to be maintained.

I think adding new features to the current @main annotation should be motivated by explaining why and how the current behavior is problematic.

[bishabosha]

We should instead replace @main with this annotaiton right away and fix any regressions as they are reported IMO.

this would mean making @main experimental because it would extend the experimental MainAnnotation

[nicolasstucki]

rebased