Implement findFirst in ReadableCollection

[laurentmuller]

The ReadableCollection has the findFirst method. This method is not defined in the Collections.phpstub file.

It's just matter as add the follwing code (copied from ReadableCollection):

/**
 * Returns the first element of this collection that satisfies the predicate p.
 *
 * @param Closure $p The predicate.
 * @psalm-param Closure(TKey, T):bool $p
 *
 * @return mixed The first element respecting the predicate, null if no element respects the predicate.
 * @psalm-return T|null
 */
public function findFirst(Closure $p);