LibGUI: Disallow selection identation in readonly text editors

Author: https://github.com/networkException
Commit: 6697ab33dc
Pull-request: https://github.com/SerenityOS/serenity/pull/14538

Userland/Libraries/LibGUI/TextEditor.cpp

@@ -868,6 +868,8 @@ void TextEditor::keydown_event(KeyEvent& event)
     }
 
     if (event.key() == KeyCode::Key_Tab) {
+        if (!is_editable())
+            return;
         if (has_selection()) {
             if (event.modifiers() == Mod_Shift) {
                 unindent_selection();