Add new icons

[phix-o]

This PR adds the following icons:

• skip-forward
• skip-backward

[reinink]

Hey! Thank you so much for this but unfortunately we aren't accepting community contributions for actual icon designs — this is Steve's pet creative project and he'd like to design the icons himself.

You can request new icons in the discussions section, here: https://github.com/tailwindlabs/heroicons/discussions

Hope that's understandable and thanks again ❤️