refactor: redesign module system

src/modules/README.md

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+## 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
+}
+```

src/modules/i18n.ts

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+import { createI18n } from 'vue-i18n'
+import { UserModule } from '/~/types'
+
+// import i18n resources
+// https://vitejs.dev/guide/features.html#glob-import
+const messages = Object.fromEntries(
+  Object.entries(
+    import.meta.globEager('../../locales/*.json'))
+    .map(([key, value]) => {
+      return [key.slice(14, -5), value.default]
+    }),
+)
+
+export const install: UserModule = ({ app }) => {
+  const i18n = createI18n({
+    legacy: false,
+    locale: 'en',
+    messages,
+  })
+
+  app.use(i18n)
+}

src/modules/nprogress.ts

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+import NProgress from 'nprogress'
+import { UserModule } from '/~/types'
+
+export const install: UserModule = ({ isClient, router }) => {
+  if (isClient) {
+    router.beforeEach(() => { NProgress.start() })
+    router.afterEach(() => { NProgress.done() })
+  }
+}