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

 # 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

plugins/nvm/_nvm

 #compdef nvm
 #autoload
 
-[[ -s ~/.nvm/nvm.sh ]] || return 0
+[[ -f "$NVM_DIR/nvm.sh" ]] || return 0
 
 local -a _1st_arguments
 _1st_arguments=(

plugins/nvm/nvm.plugin.zsh

-# The addition 'nvm install' attempts in ~/.profile
+# Set NVM_DIR if it isn't already defined
+[[ -z "$NVM_DIR" ]] && export NVM_DIR="$HOME/.nvm"
 
-[[ -s ~/.nvm/nvm.sh ]] && . ~/.nvm/nvm.sh
+# Load nvm if it exists
+[[ -f "$NVM_DIR/nvm.sh" ]] && source "$NVM_DIR/nvm.sh"