feat(template): add types for theme config (#3122)

Co-authored-by: Divyansh Singh <40380293+brc-dd@users.noreply.github.com>

template/.vitepress/theme/index.js

@@ -1,25 +1,29 @@
 // https://vitepress.dev/guide/custom-theme
-<% if (!defaultTheme) { %>import Layout from './Layout.vue'
+<% if (!defaultTheme) { %>import Layout from './Layout.vue'<% if (useTs) { %>
+import type { Theme } from 'vitepress'<% } %>
 import './style.css'
 
-export default {
+<% if (!useTs) { %>/** @type {import('vitepress').Theme} */
+<% } %>export default {
   Layout,
   enhanceApp({ app, router, siteData }) {
     // ...
   }
-}
+}<% if (useTs) { %> satisfies Theme<% } %>
-<% } else { %>import { h } from 'vue'
+<% } else { %>import { h } from 'vue'<% if (useTs) { %>
-import Theme from 'vitepress/theme'
+import type { Theme } from 'vitepress'<% } %>
+import DefaultTheme from 'vitepress/theme'
 import './style.css'
 
-export default {
+<% if (!useTs) { %>/** @type {import('vitepress').Theme} */
-  extends: Theme,
+<% } %>export default {
+  extends: DefaultTheme,
   Layout: () => {
-    return h(Theme.Layout, null, {
+    return h(DefaultTheme.Layout, null, {
       // https://vitepress.dev/guide/extending-default-theme#layout-slots
     })
   },
   enhanceApp({ app, router, siteData }) {
     // ...
   }
-}<% } %>
+}<% if (useTs) { %> satisfies Theme<% } %><% } %>