feat(gcx): add completion plugin for Grafana's gcx (#13950)

* feat(gcx): add completion plugin

* fix(gcx): write completion cache atomically

plugins/gcx/README.md

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+# Grafana CLI plugin
+
+This plugin adds completion for the [Grafana CLI (gcx)](https://github.com/grafana/gcx).
+
+To use it, add `gcx` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... gcx)
+```
+
+This plugin does not add any aliases.
+
+## Cache
+
+This plugin caches the completion script and automatically updates it when the
+plugin is loaded, which is usually when you start a new terminal emulator.
+
+The cache is stored at `$ZSH_CACHE_DIR/completions/_gcx`.

plugins/gcx/gcx.plugin.zsh

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