nvm: use nvm current in nvm_prompt_info and look in alternate install locations

This makes it work regardless of where nvm is loaded from. And it uses nvm's
version strings, which distinguish the "system" and "none" NVM environments,
instead of reporting the specific version of the system node.js or erroring,
respectively.

Fixes #4336
Closes #4338

lib/nvm.zsh

@@ -1,9 +1,8 @@
-# get the node.js version
+# get the nvm-controlled node.js version
 function nvm_prompt_info() {
-  [[ -f "$NVM_DIR/nvm.sh" ]] || return
   local nvm_prompt
-  nvm_prompt=$(node -v 2>/dev/null)
-  [[ "${nvm_prompt}x" == "x" ]] && return
-  nvm_prompt=${nvm_prompt:1}
+  which nvm &>/dev/null || return
+  nvm_prompt=$(nvm current)
+  nvm_prompt=${nvm_prompt#v}
   echo "${ZSH_THEME_NVM_PROMPT_PREFIX}${nvm_prompt}${ZSH_THEME_NVM_PROMPT_SUFFIX}"
 }