isset() narrows string-key in int-keyed-array to numeric-string

[staabm]

initial steps to make isset() implementation even more similar to array_key_exists so we can in the end hopefully share the implementation

commit 1: plain 1:1 refactoring, no implementation changes
commit 2: the fix as the PR topic suggests

[phpstan-bot]

You've opened the pull request against the latest branch 2.0.x. PHPStan 2.0 is not going to be released for months. If your code is relevant on 1.12.x and you want it to be released sooner, please rebase your pull request and change its target to 1.12.x.

[staabm]

as a first step I think its easier to have a static method for this.

after we get more similar with the array_key_exists impl we will see whether it makes sense to move this method into the ConstantScalarType-interface, as we discussed in dresden

[staabm]

this assertion is the interessting one, which got more precise with this PR

[staabm]

I am not sure whether this CI errors are related to my PR, or whether these are triggered by recent BC breaks on the 2.x branch.

-> it seems to be caused by some github infrastructure thing -> works again

[phpstan-bot]

This pull request has been marked as ready for review.

[ondrejmirtes]

Thank you!