Add better collection stubs

[Daanra]

Adds better stubs for Collection methods such that Larastan understands them better.

Larastan will now understand that e.g. the following things are not valid:

User::all()->each(function (int $user, User $key): void { // Expects User as first parameter and int as second

});

/**
  * @param EloquentCollection<User>
  * @return Collection<User>    // Collection<User> and Collection<string> mismatch
  */
public function getNames($items)
{
   return $items->map(function (User $user): string {
       return $user->name;
  });
}

[nunomaduro]

@Daanra Can you pull request an entry just bellow Unreleased on the changelog?

[canvural]

Thank you!

I updated the changelog and released 0.5.7 👍