feat: provide build end hook

[userquin]

should also be included on next ;)

closes #706

[userquin]

It seems Vitepress is not resolving the root Vite config, only expose the inlined Vite configuration and not resolving it via Vite resolveConfig.

I need to do this to make vite-plugin-pwa work (accessing the Vite plugin on buildEnd doesn't work):

import { defineConfig } from 'vitepress'
import { resolveConfig } from 'vite'
import type { VitePluginPWAAPI } from 'vite-plugin-pwa'
import { VitePWA } from 'vite-plugin-pwa'

const PWA = VitePWA({...})
...
export default defineConfig({
  lang: 'en-US',
  ...
  // add the plugin also to allow run on dev
  vite: {
    plugins: [PWA],
  },
  async buildEnd() {
      const viteConfig = await resolveConfig({
          plugins: [PWA],
        },
        'build',
        'production',
      )
      const pwaPlugin: VitePluginPWAAPI = viteConfig.plugins.find(i => i.name === 'vite-plugin-pwa')?.api
      const pwa = pwaPlugin && !pwaPlugin.disabled
      await optimizePages(pwa)
      if (pwa)
        await pwaPlugin.generateSW()
    },
})