Replace FontAwesome icons JavaScript with module

The BuildJS process used by Hugo can't tree-shake the `index.js` file
currently used. Replace it with a tree-shakable `index.mjs` file to
reduce page size.

assets/scripts/fa-icons.js

@@ -17,7 +17,7 @@ import {
   faUserAstronaut,
   faVideo,
   faW,
-} from "./pro-duotone-svg-icons";
+} from "./pro-duotone-svg-icons/index.mjs";
 import {
   faDiscord,
   faGithub,