How to find the list of icons that can be used in mac cli applications
The Terminal available in macOS is capable of displaying the Unicode encoded Emoji that are available on the system.

You can see a great list of these over at Emojipedia. While these are noted as being from iOS, generally Apple keeps the supported version of Unicode in sync between iOS and macOS.

I also found some great information about working with Emoji on macOS on NSHipster written by Mattt.