Add knip linter (#36442)

This adds [knip](https://github.com/webpro-nl/knip), a tool to find
unused files, dependencies and exports in JS. Fixed all discovered
issues.

1. knip apparently has some issue resolving imports from `d.ts` to `.ts`
so I worked around it by moving the two affected types to where they are
used.
2. I don't know why `modules/fomantic/dropdown.ts` had a new typescript
error, but I fixed it.
3. Use named export for `EsbuildPlugin`, I think this was added
recently.

web_src/js/features/tribute.ts

@@ -29,7 +29,7 @@ export async function attachTribute(element: HTMLElement) {
         return html`<div class="tribute-item">${htmlRaw(emojiHTML(item.original))}<span>${item.original}</span></div>`;
       },
     }, { // mentions
-      values: window.config.mentionValues ?? [],
+      values: window.config.mentionValues,
       requireLeadingSpace: true,
       menuItemTemplate: (item: TributeItem) => {
         const fullNameHtml = item.original.fullname && item.original.fullname !== '' ? html`<span class="fullname">${item.original.fullname}</span>` : '';