fix: DirectiveAttributes type

[HiDeoo]

Description

This PR updates the DirectiveAttributes type definition to match the one from mdast-util-directive.

Added a comment with links to explain why we can update the type without changing the implementation, as mdast-util-directive omits null and undefined values while still allowing them in the type definition.

[codspeed]

Merging this PR will not alter performance

✅ 24 untouched benchmarks
⏩ 8 skipped benchmarks¹

---

_{Comparing HiDeoo:hd/fix-directive-attributes-type (8de6512) with main (6bcdf06)}

Footnotes

1. 8 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[Princesseuh]

Looks great, thank you!