Refactor syntax highlighting infrastructure and related classes.

- Moved all language highlighter implementations (`CppHighlighter`, `GoHighlighter`, `JsonHighlighter`, etc.), the engine, and registry to `syntax/`.

Buffer.cc

@@ -7,8 +7,8 @@
 #include "UndoSystem.h"
 #include "UndoTree.h"
 // For reconstructing highlighter state on copies
-#include "HighlighterRegistry.h"
-#include "NullHighlighter.h"
+#include "syntax/HighlighterRegistry.h"
+#include "syntax/NullHighlighter.h"
 
 
 Buffer::Buffer()
@@ -542,4 +542,4 @@ const UndoSystem *
 Buffer::Undo() const
 {
 	return undo_sys_.get();
-}
+}