fix: pass appropriate workspace args and improve tests

[pi0]

🔗 Linked issue

Reported from https://github.com/nuxt/cli/issues/116

❓ Type of change

• [ ] 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• [x] 🐞 Bug fix (a non-breaking change that fixes an issue)
• [x] 👌 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

I had missed this from big PR in https://github.com/unjs/nypm/pull/75

We were only testing installations in workspace mode (and still passing wrong arguments!)

This PR, first splits tests and adds both workspace and non workspace fixtures and tests, completely separated.

Also adds support for workspace?: string|boolean. When true or "" is passed, we assume installing into the workspace root. The string value when nonempty, is the workspace package name (in class of yarn classic, dir!)

📝 Checklist

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

[codecov[bot]]

Codecov Report

Merging #90 (61928fa) into main (84357e0) will decrease coverage by 1.44%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #90      +/-   ##
==========================================
- Coverage   98.07%   96.63%   -1.44%     
==========================================
  Files           5        5              
  Lines         415      416       +1     
  Branches       66       60       -6     
==========================================
- Hits          407      402       -5     
- Misses          8       14       +6     

Files Changed	Coverage Δ
src/_utils.ts	94.77% <100.00%> (-3.73%)	:arrow_down:
src/types.ts	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes