Add monospace CSS var (#2566)

* add monospace typography token

* fix fallback style

* Create tall-chairs-push.md

* Update src/support/variables/typography.scss

Author: langermank

.changeset/tall-chairs-push.md

@@ -0,0 +1,5 @@
+---
+"@primer/css": patch
+---
+
+Add `monospace` CSS var to global scss var

src/support/variables/typography.scss

@@ -36,7 +36,7 @@ $body-font: -apple-system, BlinkMacSystemFont, 'Segoe UI', 'Noto Sans', Helvetic
 
 // Monospace font stack
 // Note: SFMono-Regular needs to come before SF Mono to fix an older version of the font in Chrome
-$mono-font: ui-monospace, SFMono-Regular, SF Mono, Menlo, Consolas, Liberation Mono, monospace !default;
+$mono-font: var(--fontStack-monospace, ui-monospace, SFMono-Regular, SF Mono, Menlo, Consolas, Liberation Mono, monospace) !default;
 
 // The base body size
 $body-font-size: 14px !default;