feat: add multi sidebar support (#38)

[kiaking]

close #38

This PR adds multi sidebar support. It does only support nesting array of objects as currently that is the only format supported by the array sidebar.

{
  sidebar: {
    '/guide/': [
      { text: 'Getting Started', link: '/guide/getting-started' },
      ...
    ],

    '/guide/nested/': [
      { text: 'Nested Index', link: '/guide/nested/' },
      ...
    ],

    // Fallback supported too.
    '/': [
      { text: 'Index', link: '/' },
      { text: 'Root', link: '/root' }
    ]
  }
}

• children property in each object works as well.
• Active state of Navbar item is not working correctly. It needs to compare correct path but thinking to create another PR for it.
• URL without trailing slash acts differently than VuePress. Currently, if we access /guide, it renders /guide.html. If we access /guide/, it renders /guide/index.html. I'm leaving it as is for now but maybe we should have follow up PR for it...?

[antfu]

I think resolvePath is a bit unclear here, would it better to be getPathDirname or something?

[kiaking]

Good point! Let me change this one

[kiaking]

Done 🙌