nvm plugin improvements (#5265)

* Export $NVM_DIR if it doesn't exist and use it when looking for nvm
* Use $NVM_DIR when looking for nvm for completion
* Use $NVM_DIR when looking for nvm in nvm_prompt_info

lib/nvm.zsh

@@ -1,6 +1,6 @@
 # get the node.js version
 function nvm_prompt_info() {
-  [ -f "$HOME/.nvm/nvm.sh" ] || return
+  [[ -f "$NVM_DIR/nvm.sh" ]] || return
   local nvm_prompt
   nvm_prompt=$(node -v 2>/dev/null)
   [[ "${nvm_prompt}x" == "x" ]] && return