✨(demo) add app demo

This demo is useful to test in the real usage of the design system as a real
user that will generate its own customized design tokens, implements the design
system's components, etc ...

apps/demo/src/cunningham-tokens.css

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+:root {
+	--c--colors--primary: purple;
+	--c--colors--secondary: #DA0000;
+	--c--text--primary: purple;
+	--c--text--secondary: #DA0000;
+}

apps/demo/src/index.scss

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+@import "cunningham-tokens";
+@import "@openfun/cunningham-react/style";
+
+:root {
+  font-family: Roboto, sans-serif;
+  font-size: 16px;
+  line-height: 24px;
+  font-weight: 400;
+  color: var(--c--colors--primary);
+  background-color: #242424;
+  height: 100vh;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+}
+
+
+.center {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+
+  h1 {
+    color: white;
+    margin-bottom: 40px;
+  }
+}

apps/demo/src/main.tsx

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+import React from "react";
+import ReactDOM from "react-dom/client";
+import "./index.scss";
+import { Button } from "@openfun/cunningham-react";
+
+ReactDOM.createRoot(document.getElementById("root") as HTMLElement).render(
+  <React.StrictMode>
+    <div className="center">
+      <h1 className="test">Cunningham Demo.</h1>
+      <Button />
+    </div>
+  </React.StrictMode>
+);

apps/demo/src/vite-env.d.ts

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+/// <reference types="vite/client" />