Replace tribute with text-expander-element for textarea (#23985)

The completion popup now behaves now much more as expected than before
for the raw textarea:
- You can press <kbd>Tab</kbd> or <kbd>Enter</kbd> once the completion
popup is open to accept the selected item
- The menu does not close automatically when moving the cursor
- When you delete text, previously correct suggestions are shown again
- If you delete all text until the opening char (`@` or `:`) after
applying a suggestion, the popup reappears again
- Menu UI has been improved

<img width="278" alt="Screenshot 2023-04-07 at 19 43 42"
src="https://user-images.githubusercontent.com/115237/230653601-d6517b9f-0988-445e-aa57-5ebfaf5039f3.png">

package.json

@@ -13,6 +13,7 @@
     "@citation-js/plugin-software-formats": "0.6.1",
     "@claviska/jquery-minicolors": "2.3.6",
     "@github/markdown-toolbar-element": "2.1.1",
+    "@github/text-expander-element": "2.3.0",
     "@mcaptcha/vanilla-glue": "0.1.0-alpha-3",
     "@primer/octicons": "18.3.0",
     "@vue/compiler-sfc": "3.2.47",