Fix list item spacing bug on Firefox (#2717)

Author: delucis

.changeset/proud-apricots-warn.md

@@ -0,0 +1,5 @@
+---
+'@astrojs/starlight': patch
+---
+
+Fixes a list item spacing issue where line break elements (`<br>`) could receive a margin, breaking layout in Firefox

packages/starlight/style/markdown.css

@@ -24,7 +24,7 @@
 
 .sl-markdown-content
 	li
-	> :last-child:not(li, ul, ol):not(a, strong, em, del, span, input, :where(.not-content *)) {
+	> :last-child:not(li, ul, ol, a, strong, em, del, span, input, code, br, :where(.not-content *)) {
 	margin-bottom: 1.25rem;
 }