fixes phpstan/phpstan#6508
fixes phpstan/phpstan#7000
fixes phpstan/phpstan#7190
see #1299

This is (a little hacky) solution to solve isset expression specifying now.

There are two problems in specifyExpressionType for ArrayDimFetch (that cannot be solved easily I think) now.

1. Specifying ArrayDimFetch with the same type of the current value can change the type in some cases

ex. https://phpstan.org/r/941b7692-8141-4fee-a410-303706ad0abf

Class Foo {	
	/**
	 * @param 'id'|'name' $key
	 * @param array{id:int, name:string} $data
	 */
	public function test2(string $key, array $data): void {
		if (isset($data[$key])) {
			\PHPStan\Testing\assertType('array{id: int, name: string}', $data);
		}
	}
}

this case was skipped before c0bf915, but the current implementation of type specifying changes the type to 'array{id: int|string, name: int|string}'

2. revertNonNullability cannot revert the non nullability correctly.

   phpstan-src/src/Analyser/NodeScopeResolver.php

   Lines 1593 to 1607 in 58b6023

   	/**
   	* @param EnsuredNonNullabilityResultExpression[] $specifiedExpressions
   	*/
   	private function revertNonNullability(MutatingScope $scope, array $specifiedExpressions): MutatingScope
   	{
   	foreach ($specifiedExpressions as $specifiedExpressionResult) {
   	$scope = $scope->specifyExpressionType(
   	$specifiedExpressionResult->getExpression(),
   	$specifiedExpressionResult->getOriginalType(),
   	$specifiedExpressionResult->getOriginalNativeType(),
   	);
   	}
   	return $scope;
   	}

A - (specify B) > -(specify A)> !== A

For example, we cannot know whether the offset was marked as required or not before the specification. (maybe phpstan/phpstan#7224 is related to this problem)

array{ 'optional'?: string|null } - (specify non null) > array{ 'optional': string } - (specify null) >
array{ 'optional': string|null } - (specify non null) > array{ 'optional': string } - (specify null) >

I had to skip $dimType with UnionType bd2eb4e because of these reasons (for now).