[RFC] Allow __toString() on enums

[DanielEScherzer]

https://wiki.php.net/rfc/stringable-enums

[iluuu1994]

In case you aren't aware of the previous proposal: https://wiki.php.net/rfc/auto-implement_stringable_for_string_backed_enums The feedback was mixed, and the proposal was eventually withdrawn. Ofc, the two proposals aren't equivalent but I suspect a lot of the feedback will be the same.

[stepo2]

The RFC and the provided test cases in this PR only cover unbacked and string-backed enums that implement __toString() in some way. What I am missing is 1) the future behaviour of enums (backed or not) without the __toString() method (especially string-backed ones) and 2) the behaviour of int-backed enums implementing the __toString() method. I assume they would behave unchanged or equivalent, but for the sake of completeness and ruling out every possible doubt this should be covered in RFC and tests.

[DanielEScherzer]

Replaced with #21425 based on mailing list feedback