fix: only merge plain objects

[pi0]

🔗 Linked issue

resolves #100

❓ 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 merging two objects, we were not checking if two objects are not pure object and also using Object.assign to merge even classes, which can lead to unwanted behavior such as merging two Date values.

This PR fixes this issue by adding little bit of overhead to the checks using a fork of is-plain-obj

📝 Checklist

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

[codecov]

Codecov Report

Merging #111 (06ba4d8) into main (59d0f6a) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #111   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines           79        92   +13     
  Branches        24        28    +4     
=========================================
+ Hits            79        92   +13     

Files	Coverage Δ
src/defu.ts	100.00% <100.00%> (ø)