feat(glab): add GitLab CLI completions (#14113)

Closes #13054

plugins/glab/glab.plugin.zsh

@@ -0,0 +1,18 @@
+# Autocompletion for the GitLab CLI (glab).
+if (( ! $+commands[glab] )); then
+  return
+fi
+
+# If the completion file doesn't exist yet, we need to autoload it and
+# bind it to `glab`. Otherwise, compinit will have already done that.
+if [[ ! -f "$ZSH_CACHE_DIR/completions/_glab" ]]; then
+  typeset -g -A _comps
+  autoload -Uz _glab
+  _comps[glab]=_glab
+fi
+
+zmodload -F zsh/files b:zf_mv
+() {
+  local TMPPREFIX="$ZSH_CACHE_DIR/completions/_glab"
+  zf_mv -f -- =( glab completion -s zsh ) "$TMPPREFIX"
+} &|