Match monospace font with main font

Since Fira Sans is used across crates.io, it makes sense to use
Fira Mono for monospaced text to increase consistency. After Fira Mono
there is now `ui-monospace` in the font stack, which renders as
SF Mono on macOS Safari without having to manually install SF Mono.
`ui-monospace` is part of the CSS Fonts Module Level 4 draft:
https://www.w3.org/TR/css-fonts-4/#ui-monospace-def.

Author: lunacookies

app/styles/application.module.css

@@ -11,7 +11,8 @@
     --footer-bg-color: var(--green800);
 
     --font-sans: "Fira Sans", sans-serif;
-    --font-monospace: SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace;
+    --font-monospace: "Fira Mono", ui-monospace, SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono",
+        "Courier New", monospace;
 
     --main-color: #383838;
     --main-color-light: #858585;