style: declare globals properly

By default, `typeset` defines variables locally unless in the main scope.
This is specially bad when using `omz plugin load`, which happens inside
a function, so the declared variables don't continue being defined when
the function finishes and the main scope reappears.

plugins/gh/gh.plugin.zsh

@@ -19,7 +19,7 @@ command mkdir -p "$ZSH_CACHE_DIR/completions"
 # If the completion file doesn't exist yet, we need to autoload it and
 # bind it to `gh`. Otherwise, compinit will have already done that.
 if [[ ! -f "$ZSH_CACHE_DIR/completions/_gh" ]]; then
-  declare -A _comps
+  typeset -g -A _comps
   autoload -Uz _gh
   _comps[gh]=_gh
 fi