feat: support customizing default theme via slots

Example:

```js
// .vitepress/theme/index.js
import { h } from 'vue'
import { defaultTheme } from 'vitepress'

const BaseLayout = defaultTheme.Layout

defaultTheme.Layout = () => {
  return h(BaseLayout, null, {
    'page-top-ads': () => h('h2', 'top ads!'),
    'page-bottom-ads': () => h('h2', 'bottom ads!'),
  })
}

export default defaultTheme
```

Author: yyx990803

src/client/app/exports.ts

@@ -2,7 +2,6 @@
 // so the user can do `import { useRoute, useSiteData } from 'vitepress'`
 
 // generic types
-export type { SiteData, PageData } from '/@types/shared'
 export type { Router, Route } from './router'
 
 // theme types
@@ -25,3 +24,6 @@ import { ComponentOptions } from 'vue'
 import _Debug from './components/Debug.vue'
 const Debug = _Debug as ComponentOptions
 export { Debug }
+
+// default theme
+export { default as defaultTheme } from '/@default-theme/index'

src/client/tsconfig.json

@@ -8,6 +8,7 @@
     "types": ["vite"],
     "paths": {
       "/@theme/*": ["theme-default/*"],
+      "/@default-theme/*": ["theme-default/*"],
       "/@shared/*": ["shared/*"],
       "/@types/*": ["../../types/*"],
       "vitepress": ["app/exports.ts"]

src/node/config.ts

@@ -2,7 +2,7 @@ import path from 'path'
 import fs from 'fs-extra'
 import chalk from 'chalk'
 import globby from 'globby'
-import { createResolver, APP_PATH } from './resolver'
+import { createResolver, APP_PATH, DEFAULT_THEME_PATH } from './resolver'
 import { Resolver } from 'vite'
 import { SiteData, HeadConfig, LocaleConfig } from '../../types/shared'
 import { MarkdownOptions } from './markdown/markdown'
@@ -48,7 +48,7 @@ export async function resolveConfig(
   const userThemeDir = resolve(root, 'theme')
   const themeDir = (await fs.pathExists(userThemeDir))
     ? userThemeDir
-    : path.join(__dirname, '../client/theme-default')
+    : DEFAULT_THEME_PATH
 
   const config: SiteConfig = {
     root,

src/node/resolver.ts

@@ -4,6 +4,10 @@ import { UserConfig } from './config'
 
 export const APP_PATH = path.join(__dirname, '../client/app')
 export const SHARED_PATH = path.join(__dirname, '../client/shared')
+export const DEFAULT_THEME_PATH = path.join(
+  __dirname,
+  '../client/theme-default'
+)
 
 // special virtual file
 // we can't directly import '/@siteData' becase
@@ -25,6 +29,7 @@ export function createResolver(
       ...userConfig.alias,
       '/@app/': APP_PATH,
       '/@theme/': themeDir,
+      '/@default-theme/': DEFAULT_THEME_PATH,
       '/@shared/': SHARED_PATH,
       vitepress: '/@app/exports.js',
       [SITE_DATA_ID]: SITE_DATA_REQUEST_PATH

types/index.d.ts

@@ -2,3 +2,4 @@ export * from './shared'
 export * from '../dist/node/index'
 export * from '../dist/client/app/exports'
 export * from '../dist/client/theme-default/config'
+export { default as defaultTheme } from '../dist/client/theme-default/index'