feat(build): add support for custom languages

[JuanM04]

Fixes #1533

[helloanoop]

Just when I was looking for this feature, I see this PR. Would be great to have this functionality merged.
@JuanM04 I have a question, How will we pass the textmate grammar in the config. Can you share a code example ?

[JuanM04]

@helloanoop Following the example in the shiki documentation, it would end up like this:

// .vitepress/config.js

import {readFileSync} from "fs"

// You don't have to have another file with your grammar, but it's convenient
const myLanguageGrammar = JSON.parse(readFileSync("./path/to/lang.tmLanguage.json"))

const myLanguage = {
  id: "mylanguage",
  scopeName: 'source.mylanguage',
  grammar: myLanguageGrammar,
  aliases: ['my', 'mylang'],
}

export default {
  markdown: {
    languages: [myLanguage]
  }
}

[helloanoop]

Awesome. Thanks @brc-dd for merging this! Been waiting to use custom textmate grammar based styling for the Bru Lang that I am working on.
https://docs.usebruno.com/bru-language-design.html