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.

templates/base/head_script.tmpl

@@ -28,6 +28,8 @@ If you introduce mistakes in it, Gitea JavaScript code wouldn't run correctly.
 				['{{$.MentionableTeamsOrg}}/{{.Name}}', {key: '{{$.MentionableTeamsOrg}}/{{.Name}}', value: '{{$.MentionableTeamsOrg}}/{{.Name}}', name: '{{$.MentionableTeamsOrg}}/{{.Name}}', avatar: '{{$.MentionableTeamsOrgAvatar}}'}],
 			{{- end -}}
 		]).values()),
+		{{else}}
+		mentionValues: [],
 		{{end}}
 		mermaidMaxSourceCharacters: {{MermaidMaxSourceCharacters}},
 		{{/* this global i18n object should only contain general texts. for specialized texts, it should be provided inside the related modules by: (1) API response (2) HTML data-attribute (3) PageData */}}