✨(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/tsconfig.json

@@ -0,0 +1,9 @@
+{
+  "extends": "@openfun/typescript-configs/react.json",
+  "compilerOptions": {
+    "noEmit": true,
+  },
+  "include": ["src"],
+  "exclude": ["node_modules","dist"],
+  "references": [{ "path": "./tsconfig.node.json" }]
+}