Fix plugin wizard's generated filenames

Fix automatic folder naming snake_case conversion.
Use plugin.gd as a default script name as advertised.

editor/plugin_config_dialog.cpp

@@ -65,7 +65,7 @@ void PluginConfigDialog::_on_confirmed() {
 		return;
 	}
 	String ext = language->get_extension();
-	String script_name = script_edit->get_text().is_empty() ? _get_subfolder() : script_edit->get_text();
+	String script_name = script_edit->get_text().is_empty() ? "plugin" : script_edit->get_text();
 	if (script_name.get_extension() != ext) {
 		script_name += "." + ext;
 	}
@@ -140,7 +140,7 @@ void PluginConfigDialog::_on_required_text_changed() {
 }
 
 String PluginConfigDialog::_get_subfolder() {
-	return subfolder_edit->get_text().is_empty() ? name_edit->get_text().replace(" ", "_").to_lower() : subfolder_edit->get_text();
+	return subfolder_edit->get_text().is_empty() ? name_edit->get_text().to_snake_case() : subfolder_edit->get_text();
 }
 
 String PluginConfigDialog::_to_absolute_plugin_path(const String &p_plugin_name) {