Add `frontend-shared` component source to tailwind `content`

To be able to use `frontend-shared` components that use Tailwind
utility classes, we need to let Tailwind know not to strip classes used
in the frontend-shared package source.

tailwind.config.mjs

@@ -6,6 +6,7 @@ export default {
     './src/sidebar/components/**/*.js',
     './src/annotator/components/**/*.js',
     './dev-server/ui-playground/components/**/*.js',
+    './node_modules/@hypothesis/frontend-shared/lib/**/*.js',
   ],
   theme: {},
   corePlugins: {