node: Optimize completions for loading lazily on demand

Move `grunt` and `gulp` to separate completion definitions wrapping
`grunt --completion=zsh` and `gulp --completion=zsh` respectively.

Since the completions are loaded lazily on demand, they avoid the
performance overhead during Zsh initialization.

Additionally, remove `npm` completion since it is already bundled with
Zsh for quite a while.

modules/node/README.md

@@ -3,6 +3,9 @@
 Provides utility functions for [Node.js][1], loads the Node Version Manager, and
 enables [_npm_][2] completion.
 
+This module must be loaded _before_ the _`completion`_ module so that the
+provided completion definitions are loaded.
+
 ## nodenv
 
 [_nodenv_][5] does one thing well - it is concerned solely with switching