Add String.is_valid_unicode_identifier()

- Adds `is_valid_unicode_identifier()`
- Adds `is_valid_ascii_identifier()`
- Deprecates `is_valid_identifier()`
- Renames `validate_identifier()` to `validate_ascii_identifier()`

editor/project_settings_editor.cpp

@@ -221,7 +221,7 @@ void ProjectSettingsEditor::_update_property_box() {
 
 		const Vector<String> names = name.split("/");
 		for (int i = 0; i < names.size(); i++) {
-			if (!names[i].is_valid_identifier()) {
+			if (!names[i].is_valid_ascii_identifier()) {
 				return;
 			}
 		}