feat(placeholder): add placeholder slot

[danekslama]

🔗 Linked issue

resolves #1307

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality)
• ✨ 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

This PR add a placeholder slot for NuxtImg component

[Baroshem]

Nice work! :)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 69.23077% with 4 lines in your changes missing coverage. Please review.

Project coverage is 62.78%. Comparing base (a2db479) to head (66a9a6b).

Files with missing lines	Patch %	Lines
src/runtime/components/NuxtImg.vue	66.66%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1626      +/-   ##
==========================================
- Coverage   62.87%   62.78%   -0.09%     
==========================================
  Files          79       79              
  Lines        3539     3547       +8     
  Branches      413      413              
==========================================
+ Hits         2225     2227       +2     
- Misses       1285     1291       +6     
  Partials       29       29              

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[danekslama]

I added the custom prop which acts similarly to the NuxtLink custom prop. I also updated the docs to show how to create custom placeholders with it.

[danielroe]

Great - this is very promising!

Would you be able to add some tests? 🙏

[danekslama]

I'll be honest, I'm fairly new to unit testing, but I have some experience with Cypress. I added a test to check if the placeholder functionality and attribute fallthrough work as expected when the custom prop is set, and it seems they do.