HackStudio: Support opening the same file in both editors

Hey, it actually works! You can now edit the same file in both editors
and even the C++ highlighting updates correctly in both of them. :^)

DevTools/HackStudio/Project.h

@@ -14,6 +14,8 @@ public:
 
     [[nodiscard]] bool add_file(const String& filename);
 
+    TextDocument* get_file(const String& filename);
+
     GModel& model() { return *m_model; }
 
     template<typename Callback>