zsh plugin to add gh command completion

gh.plugin.zsh

# TODO: 2021-12-28: remove this bit of code as it exists in oh-my-zsh.sh
# Add completions folder in $ZSH_CACHE_DIR
command mkdir -p "$ZSH_CACHE_DIR/completions"
(( ${fpath[(Ie)"$ZSH_CACHE_DIR/completions"]} )) || fpath=("$ZSH_CACHE_DIR/completions" $fpath)

# If the completion file does not exist, generate it and then source it
# Otherwise, source it and regenerate in the background
if [[ ! -f "$ZSH_CACHE_DIR/completions/_gh" ]]; then
  gh completion -s zsh  >| "$ZSH_CACHE_DIR/completions/_gh"
  source "$ZSH_CACHE_DIR/completions/_gh"
else
  source "$ZSH_CACHE_DIR/completions/_gh"
  gh completion -s zsh  >| "$ZSH_CACHE_DIR/completions/_gh" &|
fi