🐛 Add missing extension to import/export statements

A package with type: module requires extensions in the imports to
fully resolve them (tested with Webpack 5). We can simply use the .js
extension even in .ts files, tsc understands it (even though this looks
really weird).

js/src/index.ts

@@ -1 +1 @@
-export {loadCookieGroups, showCookieBar} from './cookiebar';
+export {loadCookieGroups, showCookieBar} from './cookiebar.js';