fix(evlog): support primitive values in defineEnricher

[payton-burr]

🔗 Linked issue

📚 Description

The docs show defineEnricher<string> as a supported pattern for custom enrichers, but the current implementation only accepts object types:

• EnricherDefinition<T extends object> and defineEnricher<T extends object> prevent primitive type parameters at the type level.
• mergeEventField<T extends object> was written around object spreading and didn't cleanly handle primitive computed values.

This change aligns the implementation with the documented behavior:

• Remove the T extends object constraint on EnricherDefinition, defineEnricher, and mergeEventField.
• Rework mergeEventField to handle primitives; object-merge semantics are unchanged.
• Add tests covering primitive merge paths and primitive enrichers.

📝 Checklist

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

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[github-actions]

Thank you for following the naming conventions! 🙏

[pkg-pr-new]

npm i https://pkg.pr.new/evlog@320

npm i https://pkg.pr.new/@evlog/nuxthub@320

commit: 96ff13c