Add a script for updating icon

[cxa]

For the lazy users.

p.s. if you want to test this script before merging, just use

curl -fsSL https://github.com/cxa/emacs-liquid-glass-icons/raw/refs/heads/main/patch-emacs-icon.sh | bash -s -- LG1 /Applications/Emacs.app

(replaced jimeh with cxa since this url is not available before merging).