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.

package.json

@@ -98,6 +98,7 @@
     "globals": "17.1.0",
     "happy-dom": "20.3.7",
     "jiti": "2.6.1",
+    "knip": "5.82.1",
     "markdownlint-cli": "0.47.0",
     "material-icon-theme": "5.31.0",
     "nolyfill": "1.0.44",