Add new @psalm-allow-private-mutation annotation

[christian-kolb]

Pull request to solve #2729.

@muglug As discussed here is the documentation part of the annotation. I tried to make it as similar to the other annotations as possible. Let me know if I should change anything there.
I'm very curious to see how you'll implement the annotation. Hopefully I will understand it with this example, then I might be able to offer the implementation for further annotations in the future myself 🙂
Thanks again for your offer to do the implementation for this one!

[muglug]

Thanks! I decided that this should just be a modifier for @readonly properties, as those semantics were already well-defined within Psalm, and understood externally.

Adding @psalm-allow-private-mutation to a property with a @readonly annotation prevents allows it to be edited within the class.

Another alternative to this would be @psalm-readonly-allow-private-mutation, which would combine the two. In fact I think I'll document that myself.

[muglug]

Added in b3f6b56

[christian-kolb]

Thanks! I decided that this should just be a modifier for @readonly properties, as those semantics were already well-defined within Psalm, and understood externally.

Adding @psalm-allow-private-mutation to a property with a @readonly annotation prevents allows it to be edited within the class.

Another alternative to this would be @psalm-readonly-allow-private-mutation, which would combine the two. In fact I think I'll document that myself.

@muglug Yes, that makes so much more sense 🙂 Thanks for the implementation