nvm: add Homebrew's nvm setup (#8316)

Co-authored-by: Marc Cornellà <marc.cornella@live.com>

plugins/nvm/README.md

 # nvm plugin
 
-This plugin adds autocompletions for [nvm](https://github.com/creationix/nvm) — a Node.js version manager.
+This plugin adds autocompletions for [nvm](https://github.com/nvm-sh/nvm) — a Node.js version manager.
 It also automatically sources nvm, so you don't need to do it manually in your `.zshrc`.
 
 To use it, add `nvm` to the plugins array of your zshrc file:
+
 ```zsh
 plugins=(... nvm)
 ```
+
+## Settings
+
+- **`NVM_DIR`**: if you have installed nvm in a directory other than `$HOME/.nvm`, set and export `NVM_DIR`
+  to be the directory where you installed nvm.
+  
+- **`NVM_HOMEBREW`**: if you installed nvm via Homebrew, in a directory other than `/usr/local/opt/nvm`, you
+  can set `NVM_HOMEBREW` to be the directory where you installed it.

plugins/nvm/nvm.plugin.zsh

 # Set NVM_DIR if it isn't already defined
 [[ -z "$NVM_DIR" ]] && export NVM_DIR="$HOME/.nvm"
 
-# Try to load nvm only if command not already available
-if ! type "nvm" &> /dev/null; then
-    # Load nvm if it exists
-    [[ -f "$NVM_DIR/nvm.sh" ]] && source "$NVM_DIR/nvm.sh"
+# Don't try to load nvm if command already available
+type "nvm" &> /dev/null && return
+
+# Load nvm if it exists in $NVM_DIR
+if [[ -f "$NVM_DIR/nvm.sh" ]]; then
+    source "$NVM_DIR/nvm.sh"
+    return
+fi
+
+# Otherwise try to load nvm installed via Homebrew
+
+# User can set this if they have an unusual Homebrew setup
+NVM_HOMEBREW="${NVM_HOMEBREW:-/usr/local/opt/nvm}"
+# Load nvm from Homebrew location if it exists
+[[ -f "$NVM_HOMEBREW/nvm.sh" ]] && source "$NVM_HOMEBREW/nvm.sh"
+# Load nvm bash completion from Homebrew if it exists
+if [[ -f "$NVM_HOMEBREW/etc/bash_completion.d/nvm" ]]; then
+    autoload -U +X bashcompinit && bashcompinit
+    source "$NVM_HOMEBREW/etc/bash_completion.d/nvm"
 fi