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/GitFilesView.h

@@ -13,7 +13,7 @@
 namespace HackStudio {
 
 // A "GitFileAction" is either the staging or the unstaging of a file.
-typedef Function<void(const LexicalPath& file)> GitFileActionCallback;
+using GitFileActionCallback = Function<void(const LexicalPath& file)>;
 
 class GitFilesView : public GUI::ListView {
     C_OBJECT(GitFilesView)