feat(components)!: rename nullify modifier to nullable and add optional

[sandros94]

🔗 Linked issue

Resolves: N/A

❓ 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 mainly focuses on using these components in forms with validation libraries.

Considering that most validation libraries refers to value that are T | null as nullable I decided to rename the modifier (taking advantage of the major), while also adding an optional modifier to accept undefined.

Currently upstream's NumberFieldRoot only accept optional values but not nullable ones. At first I though about a type inconsistency but noticed a strange pattern in tests when I was forcing it, decided to only implement optional for now.

What other components might take advantage of this other than UInput, UTextarea and UInputNumber?

📝 Checklist

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

[pkg-pr-new]

npm i https://pkg.pr.new/@nuxt/ui@4838

commit: 389d20c

[benjamincanac]

@sandros94 Could you add a note about this in the v4 migration guide: https://github.com/nuxt/ui/blob/v4/docs/content/docs/1.getting-started/3.migration/1.v4.md?