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/utils/match.ts

@@ -35,7 +35,7 @@ export function matchMention(queryText: string): MentionSuggestion[] {
 
   // results is a map of weights, lower is better
   const results = new Map<MentionSuggestion, number>();
-  for (const obj of window.config.mentionValues ?? []) {
+  for (const obj of window.config.mentionValues) {
     const index = obj.key.toLowerCase().indexOf(query);
     if (index === -1) continue;
     const existing = results.get(obj);