[13.x] Improve Collection::select generic types

[timacdonald]

The generic types do not satisfy this method correctly and fail on the basic documented way of using it, e.g.,

$users = collect([
    ['name' => 'Taylor Otwell', 'role' => 'Developer', 'status' => 'active'],
    ['name' => 'Victoria Faith', 'role' => 'Researcher', 'status' => 'active'],
]);
 
$users->select(['name', 'role']);

The method was expecting TKey, but that would mean it is expect a key from the parent collection, e.g., 0 and / or 1, but what we really want is a key from the nested value.

I have no idea how to do that, so I'm just making it actually work and make sense, e.g., pass a list of strings.