feat: support postcss loader

[pi0]

🔗 Linked issue

Resolves #166

❓ Type of change

• 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• 🐞 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)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

TODO: Before introducing this feature i am thinking to make the plugins configurable and (some?) opt-in.

Beta testing: Add this to package.json and run install command. It should work directly for mkdist and also for unbuild, nuxt-module-builder, etc.

  "resolutions": {
    "mkdist": "npm:@pi0/mkdist@1.4.0-rc.2"
  },

Thoughts welcome 👍🏼

📝 Checklist

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

[danielroe]

This looks great to me!

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (e7d3ffb) 95.99% compared to head (145b6e8) 96.06%.

❗ Current head 145b6e8 differs from pull request most recent head c31635d. Consider uploading reports for the commit c31635d to get more accurate results

Files	Patch %	Lines
src/loaders/index.ts	83.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #167      +/-   ##
==========================================
+ Coverage   95.99%   96.06%   +0.06%     
==========================================
  Files           8        9       +1     
  Lines         574      635      +61     
  Branches      102      115      +13     
==========================================
+ Hits          551      610      +59     
- Misses         23       25       +2     

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