Add toasts to UI (#25449)

Fixes https://github.com/go-gitea/gitea/issues/24353

In some case like async success/error, it is useful to show toasts in UI.

web_src/js/standalone/devtest.js

@@ -0,0 +1,11 @@
+import {showInfoToast, showWarningToast, showErrorToast} from '../modules/toast.js';
+
+document.getElementById('info-toast').addEventListener('click', () => {
+  showInfoToast('success 😀');
+});
+document.getElementById('warning-toast').addEventListener('click', () => {
+  showWarningToast('warning 😐');
+});
+document.getElementById('error-toast').addEventListener('click', () => {
+  showErrorToast('error 🙁');
+});