feat: allow to use custom syntax highlighting themes

[azat-io]

Adds the ability to import custom themes for syntax highlighting.

import { loadTheme } from 'shiki'
import { join } from 'path'

export default async () => {
    let theme = await loadTheme(join(__dirname, './my-theme.json'))
    return defineConfig({
      title: 'VitePress',

      markdown: {
        theme,
      },
     // ...
  })
}

[azat-io]

@brc-dd Should I rebase this PR to this branch #856 ?

[brc-dd]

No, keep it for main. I'll test this one out as soon as I get some time.