Support darkmode in other frameworks

[bongxin]

目前我使用vuepress默认主题时，按照element plus官方的暗黑模式配置方案，是可行的。但是切换到hope主题后，暗黑模式就失效了。

https://element-plus.org/zh-CN/guide/dark-mode.html

[Mister-Hope]

Similar things can be done with the following easily:

import { useDarkMode } from '@vuepress/helper/client' // install it first!

// in setup functions:
const darkMode = useDarkMode();

onMounted(() => {
  watch(darkMode, (value) => {
    if (value) document.docuementElement.classList.add('dark')
    else document.docuementElement.classList.remove('dark')
  }, { immediate: true })
})