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/Git/GitWidget.h

@@ -14,7 +14,7 @@
 
 namespace HackStudio {
 
-typedef Function<void(const String& original_content, const String& diff)> ViewDiffCallback;
+using ViewDiffCallback = Function<void(const String& original_content, const String& diff)>;
 
 class GitWidget final : public GUI::Widget {
     C_OBJECT(GitWidget)