AK: Make Vector<T>::{first,last}_matching() return Optional<T&>

These functions are _very_ misleading, as `first()` and `last()` return
references, but `{first,last}_matching()` return copies of the values.
This commit makes it so that they now return Optional<T&>, eliminating
the copy and the confusion.

AK/Vector.h

@@ -163,7 +163,7 @@ requires(!IsRvalueReference<T>) class Vector {
  VisibleType& last() { return at(size() - 1); }
 
  template<typename TUnaryPredicate>
- Optional<VisibleType> first_matching(TUnaryPredicate predicate) requires(!contains_reference)
+ Optional<VisibleType&> first_matching(TUnaryPredicate predicate) requires(!contains_reference)
  {
  for (size_t i = 0; i < size(); ++i) {
  if (predicate(at(i))) {
@@ -174,7 +174,7 @@ requires(!IsRvalueReference<T>) class Vector {
  }
 
  template<typename TUnaryPredicate>
- Optional<VisibleType> last_matching(TUnaryPredicate predicate) requires(!contains_reference)
+ Optional<VisibleType&> last_matching(TUnaryPredicate predicate) requires(!contains_reference)
  {
  for (ssize_t i = size() - 1; i >= 0; --i) {
  if (predicate(at(i))) {