LibGUI: Add a GitCommit SyntaxHighlighter implementation

This highlighter just syntax highlights the commented lines in your git
commit message. It could potentially be enhanced to handle the rebase
UI or other more advanced cases in the future.

Userland/Libraries/LibGUI/CMakeLists.txt

@@ -42,6 +42,8 @@ set(SOURCES
     FontPicker.cpp
     FontPickerDialogGML.h
     Frame.cpp
+    GitCommitLexer.cpp
+    GitCommitSyntaxHighlighter.cpp
     GlyphMapWidget.cpp
     GMLAutocompleteProvider.cpp
     GMLFormatter.cpp