Add internationalization feature

[kiaking]

related #628

VitePress Next don't have this feature yet. Let's implement it. While doing so, I would also like to suggest new config structure for locales option.

Currently, we must define two locales when defining multi locale site. One in app config level, and one in theme config level.

I think it's much easier to just be able to define locale on top level only.

export default {
  // Top level config for default and fallbacks.
  lang: 'en',
  title: 'VitePress',
  themeConfig: { ... }
  
  // All other locale settings.
  locales: {
    fr: {
      lang: 'fr',
      title: 'VitePress',
      themeConfig: { ... }
    },

    ja: {
      lang: 'ja',
      title: 'VitePress',
      themeConfig: { ... }
    }
  }
}

This should make typings much simpler too (maybe). It should eliminate weird situation like in #628 🤔

Also, maybe not required for 1.0.0 release, but consider #291 as well.

[euaaaio]

Yes, it’s a bit weird. When you create a custom theme with two languages, the localePath of useData() will be broken until you explicitly set themeConfig.locales.

[xfq]

FWIW, W3C has a Developing Localizable Manifests document (since .vitepress/config.js is also a manifest file).

[brc-dd]

Let's also track #902 and #955 here.

[fvsch]

Is the intent for RTL layouts to use CSS logical properties?
I could work on that for the default theme, if that helps.