allow calling mutation_free function inside a mutation_free context

[orklah]

This should fix #5874

I'm pretty lost however on all the annotations of purity/mutability that exists, which can be applied on which symbols and which annotation can be called inside which one.

For example, :

• pure can only be annotated on a method/function
• immutable can be annotated on a class
• you can call pure functions in immutable scope
• can you call external-mutation-free class inside an immutable scope?

It'd be great if someone could take the time to revamp the part of the doc that talks about all those: https://psalm.dev/docs/annotating_code/supported_annotations/#psalm-mutation-free

To explain the interactions between them. I'm pretty sure there are a lot of places with similar issues that I fixed here, partly because it's not very transparent what goes into what.