feat: add feature plugin tests and validation for feature flags

vite-layers/test/fixtures/devtools/base/app.config.ts

@@ -0,0 +1,7 @@
+export default {
+  name: 'base',
+  features: { billing: true, shared: 'base', nested: { on: true } },
+  hooks: {
+    'layers:resolved': () => {},
+  },
+}