Support SF Mono font in Firefox and Safari (take 2) (#1451)

* Update monospace font stack

* Create early-ducks-decide.md

Author: simurai

.changeset/early-ducks-decide.md

@@ -0,0 +1,5 @@
+---
+"@primer/css": minor
+---
+
+Support SF Mono font in Firefox and Safari

src/support/variables/typography.scss

@@ -35,7 +35,8 @@ $lh-default: 1.5 !default;
 $body-font: -apple-system, BlinkMacSystemFont, "Segoe UI", Helvetica, Arial, sans-serif, "Apple Color Emoji", "Segoe UI Emoji" !default;
 
 // Monospace font stack
-$mono-font: "SFMono-Regular", Consolas, "Liberation Mono", Menlo, monospace !default;
+// Note: SFMono-Regular needs to come before SF Mono to fix an older version of the font in Chrome
+$mono-font: ui-monospace, SFMono-Regular, SF Mono, Consolas, Liberation Mono, Menlo, monospace !default;
 
 // The base body size
 $body-font-size: 14px !default;