Fix list item spacing bug on Firefox

[delucis]

Description

• Firefox renders margins applied to <br> unlike other browsers.

• We had a style which unintentionally applied margin-bottom to <br> inside list items, which would cause odd spacing in lists only in Firefox. For example, in the Astro docs:

  

• This PR fixes this by excluding <br> from that style, and also adds <code> to avoid the same issue impacting list items with a last child that is an inline code element.

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[astrobot-houston]

size-limit report 📦

Path	Size
/index.html	6.93 KB (+0.02% 🔺)
/_astro/*.js	21.19 KB (0%)
/_astro/*.css	13.75 KB (-0.02% 🔽)

[HiDeoo]

Ah, it's been a while since that issue came out 😅 Tested in Firefox and this is indeed fixed.