Use clippie module to copy to clipboard (#23801)

Externalize clipboard copying to the
[clippie](https://github.com/silverwind/clippie) module which I feel I
can maintain outside this repo for shared benefit with my other
projects.

The module is feature-equivalent to the previous code and has one
improvement where it sets `aria-hidden` on the fallback textarea,
preventing screen readers from picking it up. Also it support `Array` of
`content` as well to copy multiple items at once, in case it's ever
needed.

web_src/js/features/copycontent.js

@@ -1,11 +1,11 @@
-import {copyToClipboard} from './clipboard.js';
+import {clippie} from 'clippie';
 import {showTemporaryTooltip} from '../modules/tippy.js';
 import {convertImage} from '../utils.js';
 
 const {i18n} = window.config;
 
 async function doCopy(content, btn) {
-  const success = await copyToClipboard(content);
+  const success = await clippie(content);
   showTemporaryTooltip(btn, success ? i18n.copy_success : i18n.copy_error);
 }