Add identifier completion for custom classes.

Previously, custom class would only auto-complete for types in GDScript.
This applies it to identifiers as well.

modules/gdscript/gdscript_editor.cpp

@@ -1271,6 +1271,14 @@ static void _find_identifiers(const GDScriptParser::CompletionContext &p_context
 		}
 		r_result.insert(option.display, option);
 	}
+
+	// Global classes
+	List<StringName> global_classes;
+	ScriptServer::get_global_class_list(&global_classes);
+	for (const StringName &E : global_classes) {
+		ScriptLanguage::CodeCompletionOption option(E, ScriptLanguage::CODE_COMPLETION_KIND_CLASS, ScriptLanguage::LOCATION_OTHER_USER_CODE);
+		r_result.insert(option.display, option);
+	}
 }
 
 static GDScriptCompletionIdentifier _type_from_variant(const Variant &p_value) {