Save the current script when adding a new method via signal connection

This makes it possible for external editors to pick up the changes.
Most modern editors should reload the file automatically,
but some older/lightweight editors may ask the user instead
(or only warn after trying to save in the external editor).

This closes #41283.

(cherry picked from commit 0ade686601)

editor/plugins/script_editor_plugin.h

@@ -276,6 +276,7 @@ class ScriptEditor : public PanelContainer {
 
 	String _get_debug_tooltip(const String &p_text, Node *_se);
 
+	void _save_current_script();
 	void _resave_scripts(const String &p_str);
 	void _reload_scripts();