Add React and TS to webpacker; add react-rails gem

app/javascript/components/Hello.tsx

@@ -0,0 +1,19 @@
+import * as React from "react";
+
+interface IProps {
+  name: string;
+}
+
+class Hello extends React.Component<IProps> {
+  render () {
+    const {name} = this.props;
+
+    return (
+      <React.Fragment>
+        <span>Hello {name}!</span>
+      </React.Fragment>
+    );
+  }
+}
+
+export default Hello;