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/helm/helm.plugin.zsh

@@ -14,7 +14,7 @@ command mkdir -p "$ZSH_CACHE_DIR/completions"
 # If the completion file doesn't exist yet, we need to autoload it and
 # bind it to `helm`. Otherwise, compinit will have already done that.
 if [[ ! -f "$ZSH_CACHE_DIR/completions/_helm" ]]; then
-  declare -A _comps
+  typeset -g -A _comps
   autoload -Uz _helm
   _comps[helm]=_helm
 fi