Add missing App.css file to hello world react + cra sample (microsoft#125)

Description of changes

Adds a missing App.css file back to the React + CRA hello world sample extension.

Author: hawkticehurst

frameworks/hello-world-react-cra/webview-ui/src/App.css

@@ -0,0 +1,11 @@
+main {
+  display: flex;
+  flex-direction: column;
+  justify-content: center;
+  align-items: flex-start;
+  height: 100%;
+}
+
+main > * {
+  margin: 1rem 0;
+}

frameworks/hello-world-react-cra/webview-ui/src/App.tsx

@@ -1,5 +1,6 @@
 import { vscode } from "./utilities/vscode";
 import { VSCodeButton } from "@vscode/webview-ui-toolkit/react";
+import "./App.css";
 
 function App() {
   function handleHowdyClick() {