[rules] Add NoMissingVariableDimFetchRule

[TomasVotruba]

This rule sposts dim fetch assign to a non-existing variable:

class SomeClass
{
    public function some()
    {
        $dim['key'] = 'value';
    }
}

To use it, add it to your phpstan.neon:

rules:
    - Symplify\PHPStanRules\Rules\Explicit\NoMissingVariableDimFetchRule

Have fun and happy coding 🥳