feat: esbuild config option to modify build options

[edemaine]

❓ 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

Vite has a config option that lets plugins view and/or modify the configuration.

I've been missing this option in esbuild. This PR adds a similar config option, which gets passed in build.initialOptions.

📝 Checklist

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