Polyfill the window.customElements (#23592)

Related: #23590

Reference:
https://github.com/webcomponents/polyfills/tree/master/packages/webcomponentsjs

It seems that there are some users using old browsers, so the
`window.customElements` need polyfill.

The Custom Elements would help a lot for Gitea's UI problems, including:

* `<span class="js-pretty-number">`
* `<time data-format>`

So it's worth get polyfill.

---------

Co-authored-by: delvh <dev.lh@web.de>

web_src/js/webcomponents/GiteaOriginUrl.js

@@ -1,3 +1,5 @@
+import '@webcomponents/custom-elements'; // automatically adds custom elements for older browsers that don't support it
+
 // this is a Gitea's private HTML component, it converts an absolute or relative URL to an absolute URL with the current origin
 window.customElements.define('gitea-origin-url', class extends HTMLElement {
   connectedCallback() {