✨(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/.eslintrc.json

@@ -0,0 +1,12 @@
+{
+  "root": true,
+  "extends": [
+    "custom"
+  ],
+  "parserOptions": {
+    "project": [
+      "./tsconfig.json",
+      "./tsconfig.node.json"
+    ]
+  }
+}