feat: add `defuSchema`

[ferferga]

🔗 Linked issue

Fixes #48

❓ Type of change

• [X] 📖 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)
• [X] ✨ 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

• Minor code improvement by using optional chaining as well when checking for merger function
• Create defuSchema for removing keys not present in the default object

📝 Checklist

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

[ferferga]

@pi0 Can I get a review in this please?

[ferferga]

@pi0 Can I get a re-review of this please? Would be realy useful to have this upstream already 😅

[pi0]

Once last thing, it is missing in https://github.com/unjs/defu/blob/main/lib/defu.cjs

[ferferga]

@pi0 Done!

[ferferga]

@pi0 Hey sorry for pinging again about this... But it would be really useful to have this upstream!

[ferferga]

@pi0 I fixed the conflicts after the merge of #121