Add minimum polyfill to support "relative-time-element" in PaleMoon (#26575) (#26578)

Backport #26575 by @wxiaoguang

Close #26525

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/js/webcomponents/webcomponents.js

@@ -1,3 +1,5 @@
 import '@webcomponents/custom-elements'; // polyfill for some browsers like Pale Moon
+import './polyfill.js';
+
 import '@github/relative-time-element';
 import './GiteaOriginUrl.js';