Userland: Change typedef to using directive

Problem:
- `typedef`s are read backwards making it confusing.
- `using` statements can be used in template aliases.
- `using` provides similarity to most other C++ syntax.

- C++ core guidelines say to prefer `using` over `typedef`:
  https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rt-using

Solution:
- Switch these where appropriate.

Userland/Applications/Piano/Track.h

@@ -13,7 +13,7 @@
 #include <AK/SinglyLinkedList.h>
 #include <LibAudio/Buffer.h>
 
-typedef AK::SinglyLinkedListIterator<SinglyLinkedList<RollNote>, RollNote> RollIter;
+using RollIter = AK::SinglyLinkedListIterator<SinglyLinkedList<RollNote>, RollNote>;
 
 class Track {
     AK_MAKE_NONCOPYABLE(Track);