Set markdown hr thickness to border-width (1px)

The markdown thematic break (hr) rendered at 4px via the $em-spacer-3
spacing token, roughly 4x the ~1px used by every other markdown renderer
and by Primer's own rules (headings, tables, and blockquote all use
$border-width). Use $border-width for the hr so it matches the system's
line weight and tracks the token if it ever changes.

Only the existing $border-width token is used; no new tokens or API.
Passes stylelint with no new disables.

Author: dapirian

src/markdown/markdown-body.scss

@@ -76,7 +76,7 @@
   }
 
   hr {
-    height: $em-spacer-3;
+    height: $border-width;
     padding: 0;
     margin: var(--base-size-24) 0;
     // stylelint-disable-next-line primer/colors