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/markup/refissue.ts

@@ -11,7 +11,7 @@ export function initMarkupRefIssue(el: HTMLElement) {
   });
 }
 
-export function showMarkupRefIssuePopup(e: MouseEvent | FocusEvent) {
+function showMarkupRefIssuePopup(e: MouseEvent | FocusEvent) {
   const refIssue = e.currentTarget as HTMLElement;
   if (getAttachedTippyInstance(refIssue)) return;
   if (refIssue.classList.contains('ref-external-issue')) return;