Auto merge of #3902 - arzg:fira-mono, r=Turbo87

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 Safari without having to manually install SF Mono. `ui-monospace` [is part of](https://www.w3.org/TR/css-fonts-4/#ui-monospace-def) the CSS Fonts Module Level 4 draft.

Author: bors

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;