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/DevTools/HackStudio/Debugger/BreakpointCallback.h

@@ -17,6 +17,5 @@ enum class BreakpointChange {
     Removed,
 };
 
-typedef Function<void(const String& file, size_t line, BreakpointChange)> BreakpointChangeCallback;
-
+using BreakpointChangeCallback = Function<void(const String& file, size_t line, BreakpointChange)>;
 }