fix: esbuild emitFile should mkdir subdirectories too

[edemaine]

🔗 Linked issue

n/a

❓ 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

When calling this.emitFile({fileName: 'subdir/foo.d.ts', source, type: 'asset'}), Rollup recursively makes the directory dist/subdir if necessary.

But unplugin's esbuild plugin does not; currently, it just makes the directory dist (where dist is the outdir option). If dist/subdir doesn't exit, the emit crashes.

This small PR changes the esbuild emitFile mkdir call to apply to the full directory (dist/subdir in the example, instead of dist).

📝 Checklist

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