Added placeholder inventory react component

inventory-app/src/__snapshots__/app.test.js.snap

 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
-exports[`the app has one labeled button that displays the value from the store 1`] = `<div />`;
+exports[`the app has one labeled button that displays the value from the store 1`] = `
+<div>
+  <main>
+    [placeholder text]
+  </main>
+</div>
+`;

inventory-app/src/app.js

 import { Routes, Route } from 'react-router-dom';
+import { Inventory } from './features/inventory/inventory.js';
 
 export function App() {
   const page =
     <>
+      <Inventory/>
     </>;
   return (
     <Routes>

inventory-app/src/features/inventory/inventory.js

+export function Inventory() {
+  return (
+    <main>
+      [placeholder text]
+    </main>
+  );
+}