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">

web_src/css/editor-markdown.css

@@ -30,3 +30,66 @@
 .combo-markdown-editor .CodeMirror-scroll {
   max-height: calc(100vh - 200px);
 }
+
+text-expander {
+  display: block;
+  position: relative;
+}
+
+text-expander .suggestions {
+  position: absolute;
+  min-width: 180px;
+  padding: 0;
+  margin-top: 24px;
+  list-style: none;
+  background: var(--color-box-body);
+  border-radius: 5px;
+  border: 1px solid var(--color-secondary);
+  box-shadow: 0 .5rem 1rem var(--color-shadow);
+}
+
+text-expander .suggestions li {
+  display: flex;
+  align-items: center;
+  cursor: pointer;
+  padding: 4px 8px;
+  font-weight: 500;
+}
+
+text-expander .suggestions li + li {
+  border-top: 1px solid var(--color-secondary-alpha-40);
+}
+
+text-expander .suggestions li:first-child {
+  border-radius: 4px 4px 0 0;
+}
+
+text-expander .suggestions li:last-child {
+  border-radius: 0 0 4px 4px;
+}
+
+text-expander .suggestions li:only-child {
+  border-radius: 4px;
+}
+
+text-expander .suggestions li:hover {
+  background: var(--color-hover);
+}
+
+text-expander .suggestions .fullname {
+  font-weight: normal;
+  margin-left: 4px;
+  color: var(--color-text-light-1);
+}
+
+text-expander .suggestions li[aria-selected="true"],
+text-expander .suggestions li[aria-selected="true"] span {
+  background: var(--color-primary);
+  color: var(--color-primary-contrast);
+}
+
+text-expander .suggestions img {
+  width: 24px;
+  height: 24px;
+  margin-right: 8px;
+}