feat: add feature plugin tests and validation for feature flags

vite-layers/example/apps/brand/app.config.ts

@@ -1,5 +1,8 @@
 import { defineLayerConfig } from '../../../src/index.ts'
 
+// Northwind — a reseller brand. It inherits the entire shell (header, footer, profile, router,
+// Tailwind setup) from `main` and changes only three things: its logo, its theme.css tokens and
+// its Landing page. It also drops the billing page entirely.
 export default defineLayerConfig({
   name: 'brand',
   extends: ['../main'],