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.

knip.config.ts

@@ -0,0 +1,18 @@
+import type {KnipConfig} from 'knip';
+
+export default {
+  entry: [
+    '*.ts',
+    'tools/**/*.ts',
+    'tests/e2e/**/*.ts',
+  ],
+  ignoreDependencies: [
+    // dependencies used in Makefile or tools
+    '@primer/octicons',
+    'markdownlint-cli',
+    'nolyfill',
+    'spectral-cli-bundle',
+    'vue-tsc',
+    'webpack-cli',
+  ],
+} satisfies KnipConfig;