chore(react-icons): improve build performance (#868)

* chore(react-icons): introduce 10x faster unfill alternative

* chore(react-icons): run font generation tasks in parallel

* chore(react-icons): replace slow shell unfill with unfill.js

* fixup! chore(react-icons): introduce 10x faster unfill alternative

* fixup! chore(react-icons): replace slow shell unfill with unfill.js

Author: Hotell