Add support for ::class type inference

[simPod]

Calling $class = get_class($obj); for $obj of type T correctly resolves $class to class-string<T>.

But $class = $obj::class resolves $class to mixed

https://psalm.dev/r/57434e1f10

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/57434e1f10

<?php

/**
 * @template T of object
 * @psalm-param T $obj
 */
function a(object $obj) {
    $class = get_class($obj);
    $class2 = $obj::class;
    
    /** @psalm-trace $class */
    /** @psalm-trace $class2 */
}

Psalm output (using commit b627962):

INFO: MixedAssignment - 9:5 - Unable to determine the type that $class2 is being assigned to

INFO: Trace - 12:0 - $class2: mixed

INFO: UnusedVariable - 8:5 - $class is never referenced or the value is not used

INFO: UnusedVariable - 9:5 - $class2 is never referenced or the value is not used

INFO: MissingReturnType - 7:10 - Method a does not have a return type, expecting void

[AndrolGenhald]

Ideally support would be conditioned on PHP 8, and previous versions would show an error: https://3v4l.org/ZCj6L

[simPod]

It's PHP8+ syntax.