HackStudio: Display warning when opening binary files

We now detect and display a warning when we can't render the text of an
opened file.

Closes #5062.

Userland/DevTools/HackStudio/ProjectFile.h

@@ -42,6 +42,7 @@ public:
     }
 
     const String& name() const { return m_name; }
+    bool could_render_text() const { return m_could_render_text; }
 
     GUI::TextDocument& document() const;
     CodeDocument& code_document() const;
@@ -57,6 +58,7 @@ private:
 
     String m_name;
     mutable RefPtr<CodeDocument> m_document;
+    mutable bool m_could_render_text { false };
     int m_vertical_scroll_value { 0 };
     int m_horizontal_scroll_value { 0 };
 };