Add “Inter“ as base sans-serif typeface (#236)

Before it was required to manually configure the sans-serif typeface
based on the running OS [1] due to licensing and the availability of
specific font families on different operating systems.
Since I use the fantastic Inter [2] typeface family for almost all UI,
text and designs it is now set as default sans-serif typeface.
Inter is open source and therefore system independent and is installed
in all of my systems/habitats.

[1]: https://github.com/arcticicestudio/igloo/tree/eb222438e6cd93b19c222043570bb7c627ea3e3b/snowblocks/firefox#typeface
[2]: https://rsms.me/inter

Closes GH-225

snowblocks/firefox/user.js

@@ -359,6 +359,7 @@ user_pref("findbar.highlightAll", false);
 /* Set the system independent typefaces. */
 user_pref("font.default.x-western", "sans-serif");
 user_pref("font.name.monospace.x-western", "Source Code Pro");
+user_pref("font.name.sans-serif.x-western", "Inter");
 user_pref("font.size.fixed.x-western", 16);
 
 /* Disable the fullscreen warning popup. */