feat: allow setting giget clone options

[pi0]

🔗 Linked issue

❓ 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

This PR allows setting giget cloning options both with top level loadConfig and also in each extends layer using layer meta:

export default defineConfig({
  theme: "./theme",
  extends: [
    ["gh:user/repo", { giget: { auth: process.env.GITHUB_AUTH } }],
  ]
})

Also unblocks #51 since once we support this feature in giget, (unjs/giget#6), giget: { installDependencies: true } would allow installing layer dependencies!

📝 Checklist

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

[codecov]

Codecov Report

Merging #112 (038f98b) into main (a8b73c2) will increase coverage by 0.24%.
Report is 1 commits behind head on main.
The diff coverage is 100.00%.

❗ Current head 038f98b differs from pull request most recent head af13d45. Consider uploading reports for the commit af13d45 to get more accurate results

@@            Coverage Diff             @@
##             main     #112      +/-   ##
==========================================
+ Coverage   77.93%   78.17%   +0.24%     
==========================================
  Files           5        5              
  Lines         716      724       +8     
  Branches       65       65              
==========================================
+ Hits          558      566       +8     
  Misses        158      158              

Files	Coverage Δ
src/loader.ts	90.31% <100.00%> (+0.12%)	⬆️
src/types.ts	95.53% <100.00%> (+0.16%)	⬆️