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

Closes #13054

plugins/glab/README.md

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+# GitLab CLI plugin
+
+This plugin adds completion for the [GitLab CLI](https://docs.gitlab.com/cli/).
+
+To use it, add `glab` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... glab)
+```
+
+This plugin does not add any aliases.
+
+## Cache
+
+This plugin caches the completion script at `$ZSH_CACHE_DIR/completions/_glab`.
+
+The cache is regenerated in the background whenever the plugin is loaded, which
+usually happens when you start a new Zsh session.

plugins/glab/glab.plugin.zsh

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+# 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"
+} &|