Enable more checks for native classes on PHP 8

[danog]

Coupled with #7475, aims to reach feature parity with phpstan vs psalm.

This PR enables these checks only on 8.1, and like phpstan it doesn't support suppression via the ReturnTypeWillChange attribute, which could possibly cause BC problems when seeking to support PHP 7 (unlike my earlier merged PR which does support ReturnTypeWillChange, but doesn't check for things like mismatched types).

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/66d598df84

<?php

class Foo implements Iterator
{
    public function current(string $foo) {
        return 'foo';
    }
    
    public function next($a) {
    }
    
    public function key() {}
    
    public function valid(): string
    {
        return 'foo';
    }
    
    public function rewind(): void
    {
        
    }
}

Psalm output (using commit ab2b77d):

ERROR: MethodSignatureMustProvideReturnType - 5:21 - Method Foo::current must have a return type signature!

ERROR: MethodSignatureMustProvideReturnType - 9:21 - Method Foo::next must have a return type signature!

ERROR: MethodSignatureMustProvideReturnType - 12:21 - Method Foo::key must have a return type signature!

INFO: MissingParamType - 9:26 - Parameter $a has no provided type

[danog]

All done, the checks are enabled when targeting 8.0+.

[orklah]

Thanks for the work. I'm afraid we have some issues about user_defined when using stubs (which a lot of users do) that won't allow this to be merged for now...

We had to disable your other related feature about type inheritance in the core because of that stub issue so I feat this will be compromised as well. Please keep this open, if the fix is made in the stubs, we'll be able to merge that then