refactor(definitions.typ): use single rule for definition item

[mcanouil]

Consolidate the item display rule.

Follow up on:

• fix(typst): prevent breaking inside definition items #13978

Upstream:

• refactor(typst): improve terms accessibility jgm/pandoc#11436

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[cderv]

Test are passing, and it is already merged upstream. Seems ok to merge for 1.9.

@gordonwoodhull what do you think ?

[gordonwoodhull]

Absolutely. Took me a minute to understand, because I thought we had already done this.

But this is something we discussed after merging #13978, but never implemented here.