feat(markdown): support multiple themes for code highlighter

[farnabaz]

🔗 Linked issue

resolves #1135

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

// nuxt.config
{
  content: {
    highlight: {
      theme: {
        default: 'github-light',
        dark: 'github-dark'
      }
    }
  }
}

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

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[Tahul]

LGTM :)

I've added documentation for the feature, could you review it then merge the PR @farnabaz ? 😄

[farnabaz]

@Tahul LGTM 👍

[vinayakkulkarni]

Can this be done in v2.0.2 or a new release?

[farnabaz]

@vinayakkulkarni you can use this feature by using the edge version. See https://content.nuxtjs.org/guide/migration/edge-channel