feat(zsh): add `nd` alias for `nix develop`

Signed-off-by: Seongmin Lee <[e-mail]>

zsh/.zshrc

···
 alias gs="git status"
 alias ta="tmux attach"
 alias nn="NVIM_APPNAME=nativevim nvim"
+alias nd="nix develop -c zsh -l"
 
 ## Custom prompt
 setopt prompt_subst