feat(core): validateField supports returning validation results

[Mini-ghost]

🔗 Linked issue

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 like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Before

const error = validateField('fieldName')
// The `error` is `undefined` even if validation fails

After

const error = validateField('fieldName')
// The `error` is `FieldError<Values, Name>` if validation fails

📝 Checklist

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