node: Revamp and optimize 'node' module

Changes:
- nodenv and nvm now honors (and prioritizes) `$XDG_CONFIG_HOME` over
  `$HOME` to lookup local nodenv/nvm installation.
- Make `nvm` loading lazy (via `--no-use` argument).
- Remove redundant NODENV_ROOT or NVM_DIR, respective script already
  set them up.
- Adhere to more idiomatic Zsh operation and minimize external command
  usage (like `sed`).

modules/node/functions/node-doc

@@ -13,6 +13,6 @@ if [[ -z "$BROWSER" ]]; then
 fi
 
 # TODO: Make the sections easier to use.
-"$BROWSER" "http://nodejs.org/docs/$(node --version | sed 's/-.*//')/api/all.html#${1}"
+"$BROWSER" "https://nodejs.org/docs/${$(node --version 2> /dev/null)/%-*}/api/all.html#${1}"
 
 # }