Support default user; use emojis and fallback to Nerd Font

[patrickdappollonio]

This PR performs two main changes:

gc-rust now can find if a GC_GITHUB_USERNAME or GITHUB_USERNAME is set, and if so, use that as the username of the command-line prompt. So for me, if I want to clone this repo or, say, http-server I can simply set my GITHUB_USERNAME to `patrickdappollonio and run:

# set my username
export GITHUB_USERNAME=patrickdappollonio

# to clone github.com/patrickdappollonio/gc-rust
gc-rust gc-rust 

# to clone github.com/patrickdappollonio/http-server
gc-rust http-server 

(If you're using the bash function, you need to replace gc-rust with `gc).

The second change is that several users reported issues with the Nerd Font glyphs used in their terminals. Not everyone has a patched Nerd Font set. To keep this behaviour backwards-compatible, we'll do the following:

• We'll prefer rendering emojis whenever your terminal reports emojis are supported
• We'll fallback to Nerd Font glyphs whenever possible

This makes it friendly for use on new versions of the Gnome shell, Konsole or even the default Terminal.app on macOS (or the new Ghostty).