WebViewPluginDemo: Add empty React GUI project

This commit contains an unmodified React app template. The interesting
bit is the following commit that shows how to modify this template to
interoperate with the JUCE framework.

examples/GUI/WebViewPluginDemoGUI/src/App.js

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+import "./App.css";
+
+import { React } from "react";
+
+function App() {
+  return (
+    <div className="App">
+      <header className="App-header">
+        <p>
+          Edit <code>src/App.js</code> and save to reload.
+        </p>
+        <a
+          className="App-link"
+          href="https://reactjs.org"
+          target="_blank"
+          rel="noopener noreferrer"
+        >
+          Learn React
+        </a>
+      </header>
+    </div>
+  );
+}
+
+export default App;

examples/GUI/WebViewPluginDemoGUI/src/App.test.js

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+import { render, screen } from '@testing-library/react';
+import App from './App';
+
+test('renders learn react link', () => {
+  render(<App />);
+  const linkElement = screen.getByText(/learn react/i);
+  expect(linkElement).toBeInTheDocument();
+});

examples/GUI/WebViewPluginDemoGUI/src/index.css

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+body {
+  margin: 0;
+  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', 'Roboto', 'Oxygen',
+    'Ubuntu', 'Cantarell', 'Fira Sans', 'Droid Sans', 'Helvetica Neue',
+    sans-serif;
+  -webkit-font-smoothing: antialiased;
+  -moz-osx-font-smoothing: grayscale;
+}
+
+code {
+  font-family: source-code-pro, Menlo, Monaco, Consolas, 'Courier New',
+    monospace;
+}

examples/GUI/WebViewPluginDemoGUI/src/index.js

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+import React from "react";
+import ReactDOM from "react-dom/client";
+import "./index.css";
+import App from "./App";
+import reportWebVitals from "./reportWebVitals";
+
+const root = ReactDOM.createRoot(document.getElementById("root"));
+root.render(
+  <React.StrictMode>
+    <App />
+  </React.StrictMode>
+);
+
+// If you want to start measuring performance in your app, pass a function
+// to log results (for example: reportWebVitals(console.log))
+// or send to an analytics endpoint. Learn more: https://bit.ly/CRA-vitals
+reportWebVitals();

examples/GUI/WebViewPluginDemoGUI/src/reportWebVitals.js

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+const reportWebVitals = onPerfEntry => {
+  if (onPerfEntry && onPerfEntry instanceof Function) {
+    import('web-vitals').then(({ getCLS, getFID, getFCP, getLCP, getTTFB }) => {
+      getCLS(onPerfEntry);
+      getFID(onPerfEntry);
+      getFCP(onPerfEntry);
+      getLCP(onPerfEntry);
+      getTTFB(onPerfEntry);
+    });
+  }
+};
+
+export default reportWebVitals;

examples/GUI/WebViewPluginDemoGUI/src/setupTests.js

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+// jest-dom adds custom jest matchers for asserting on DOM nodes.
+// allows you to do things like:
+// expect(element).toHaveTextContent(/react/i)
+// learn more: https://github.com/testing-library/jest-dom
+import '@testing-library/jest-dom';