feat(theme): add --vp-layout-top-height to adjust banner

[ylw5]

#1497 (comment)
Add Banner to layout-top slot similar to @vue/theme.

// .vitepress/theme/index.ts

export default {
  ...DefaultTheme,
  Layout() {
    return h(DefaultTheme.Layout, null, {
      'layout-top': () => h(Banner)
    })
  }
}

// Banner.vue

<template>
  <div class="banner">
    banner
  </div>
</template>

<style>
html{
  --vp-layout-top-height: 30px;
}
</style>

<style scoped>
.banner {
  position: fixed;
  z-index:var(--vp-z-index-layout-top);
  top: 0;
  left: 0;
  right: 0;
  height: var(--vp-layout-top-height);
  background: -webkit-linear-gradient(315deg, #96c93c 25%, #03b19a);
}
</style>

[ylw5]

#1497

[brc-dd]

Make it --vp- instead of --vt-.

[brc-dd]

Thanks! 🎉

[ylw5]

Thanks for your review!

[vedxp]

Thanks for your review!

Would it be documented any soon, sir?

[ylw5]

Would it be documented any soon, sir?

I don't know if it is necessary to add description of layout-top to document.