Add frontend tests and other stuff

frontend/index.js

@@ -0,0 +1,8 @@
+// Note that a dynamic `import` statement here is required due to
+// webpack/webpack#6615, but in theory `import { greet } from './pkg';`
+// will work here one day as well!
+const rust = import('./pkg');
+
+rust
+  .then(m => m.greet('Paul!'))
+  .catch(console.error);