refactor: redesign module system

src/modules/README.md

@@ -0,0 +1,11 @@
+## Modules
+
+A custom user module system. Place a `.ts` file with the following template, it will be installed automatically.
+
+```ts
+import { UserModule } from '/~/types'
+
+export const install: UserModule = ({ app, router, isClient }) => {
+  // do something
+}
+```