fix(i18n): fix locales reading, add site.langs

[posva]

Still need some unit/integration tests:

Tested with the following config:

module.exports = {
  // lang: 'en-US',
  // title: 'VitePress',
  // description: 'Vite & Vue powered static site generator.',

  locales: {
    '/': {
      lang: 'en-US',
      title: 'VitePress',
      description: 'Vite & Vue powered static site generator.'
    },
    '/zh/': {
      lang: 'zh-CN',
      title: '中文 Vitepress',
      description: '中文 description'
    }
  },

  themeConfig: {
    repo: 'vuejs/vitepress',
    docsDir: 'docs',
    editLinks: true,
    algolia: {
      apiKey: 'c57105e511faa5558547599f120ceeba',
      indexName: 'vitepress'
    },

    carbonAds: {
      carbon: 'CEBDT27Y',
      custom: 'CKYD62QM',
      placement: 'vuejsorg'
    },

    locales: {
      '/': {
        editLinkText: 'Edit this page on GitHub',
        lastUpdated: 'Last Updated',
        label: 'English',

        nav: [
          { text: 'Guide', link: '/', activeMatch: '^/$|^/guide/' },
          {
            text: 'Config Reference',
            link: '/config/basics',
            activeMatch: '^/config/'
          },
          {
            text: 'Release Notes',
            link: 'https://github.com/vuejs/vitepress/releases'
          }
        ],

        sidebar: {
          '/guide/': getGuideSidebar(),
          '/config/': getConfigSidebar(),
          '/': getGuideSidebar()
        }
      },
      '/zh/': {
        editLinkText: 'Edit this page on GitHub',
        lastUpdated: 'Last Updated',
        label: '中文',

        nav: [
          { text: 'Guide', link: '/zh/', activeMatch: '^/$|^/guide/' },
          {
            text: 'Release Notes',
            link: 'https://github.com/vuejs/vitepress/releases'
          }
        ],

        sidebar: {
          '/guide/': getGuideSidebar(),
          '/config/': getConfigSidebar(),
          '/': getGuideSidebar()
        }
      }
    }
  }
}

function getGuideSidebar() {
  return [
    {
      text: 'Introduction',
      children: [
        { text: 'What is VitePress?', link: '/' },
        { text: 'Getting Started', link: '/guide/getting-started' },
        { text: 'Configuration', link: '/guide/configuration' },
        { text: 'Asset Handling', link: '/guide/assets' },
        { text: 'Markdown Extensions', link: '/guide/markdown' },
        { text: 'Using Vue in Markdown', link: '/guide/using-vue' },
        { text: 'Deploying', link: '/guide/deploy' }
      ]
    },
    {
      text: 'Advanced',
      children: [
        { text: 'Frontmatter', link: '/guide/frontmatter' },
        { text: 'Theming', link: '/guide/theming' },
        { text: 'API Reference', link: '/guide/api' },
        {
          text: 'Differences from Vuepress',
          link: '/guide/differences-from-vuepress'
        }
      ]
    }
  ]
}

function getConfigSidebar() {
  return [
    {
      text: 'App Config',
      children: [{ text: 'Basics', link: '/config/basics' }]
    },
    {
      text: 'Theme Config',
      children: [
        { text: 'Homepage', link: '/config/homepage' },
        { text: 'Algolia Search', link: '/config/algolia-search' },
        { text: 'Carbon Ads', link: '/config/carbon-ads' }
      ]
    }
  ]
}