ParameterBag->keys() return type is incorrect

[nreynis]

In the ParameterBag stub the return type is improperly narrowed to list<string>.

The implementation use a simple array_keys call. Therefore the return type MUST be list<array-key>.

With the current type narrowing it's impossible to correctly parse a query string with numeric keys (ie: https://domain.tld/search?0=value&78=something). Worse, trusting PHPStan than the key is a string might lead to a runtime TypeError.