[parser] use simple parser to remove unsupported code

[TomasVotruba]

Mirror to fix in PHPStan phpstan/phpstan-src@433511a

Closes rectorphp/rector#6970

[samsonasik]

Can skipped ReservedFnFunctionRector test be enabled with it?

rector-src/rules-tests/Php74/Rector/Function_/ReservedFnFunctionRector/ReservedFnFunctionRectorTest.php

Line 18 in 7c14a98

$this->markTestSkipped('Requires tweaked PHP Parser to verify `fn` as valid function to simulate php <7.4');

[TomasVotruba]

ReservedFnFunctionRector

I've just tried it and answer is... no.
It would require to run tests on version when the fn was not reserved keyword.

[TomasVotruba]

It didn't resolve the issue yet. I'll check it tomorrow