fix(esbuild): sourcemaps in transform result can be a string

[snowystinger]

🔗 Linked issue

Brought up over here devongovett/unplugin-parcel-macros#4

❓ 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

The type cast to any on

unplugin/src/esbuild/index.ts

Line 288 in 867b71d

map = result.map as any

was hiding a bug where the result could be a string. When it was, you'd get an error such as Cannot create property 'sourcesContent' on string '{"version":3,"sources":["test.js"]

I'm not quite sure how to write a test for this, though, as I'm not entirely sure how this actually occurs other than possibly when passed across languages, ie rust to js. Any pointers are welcome.
I'd thought to change

unplugin/test/fixtures/transform/unplugin.js

Line 35 in 867b71d

map: s.generateMap({

by wrapping it in a JSON.stringify, but that didn't work.

📝 Checklist

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