feat: use combo markdown editor for milestone description

- Use the combo markdown editor for the milestone description. The
milestone description is rendered in markdown, so it makes sense to use
a 'markdown-aware' editor. This also includes the option to use
monospace font.
- Resolves #5649

web_src/js/features/repo-milestone.js

@@ -0,0 +1,9 @@
+import {initComboMarkdownEditor} from './comp/ComboMarkdownEditor.js';
+
+export function initRepoMilestoneEditor() {
+  const editor = document.querySelector('.page-content.repository.milestone .combo-markdown-editor');
+  if (!editor) {
+    return;
+  }
+  initComboMarkdownEditor(editor);
+}