Migrates away from unmaintained npm package fantasticon

Replaces with @twbs/fantasticon fork, which is maintained.

Tested by running `npm run compile-icons` (which is indirectly
included in the `postinstall` script too), and then previewing
the built font (assets/icons/icon-font.woff) on
https://fontdrop.info. The 3 expected glyphs were present at
the expected codepoints.

This does NOT resolve the npm install warning I initially was
pursuring. The transitive dep chain is deep, and @twbs/fantasticon
still contains the deprecated version of @npmcli/move-file.

> npm warn deprecated @npmcli/move-file@2.0.1: This functionality has been moved to @npmcli/fs

Author: aoberoi