feat: call hooks before and after parse

[farnabaz]

🔗 Linked issue

nuxt/framework#1119

❓ 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

Introduce two hooks to manipulate contents before/after parsing.

📝 Checklist

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

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[Atinux]

Would be nice to add a test case and document it somewhere 😊

[Tahul]

This looks good to me, will it unlock proper reloading with Tailwind classes used in .md files?

I would only add a link to https://nitro.unjs.io/guide/plugins.html somewhere in the docs and specify where these plugins should be placed. 🙂