Build Tools: Add an npm script shortcut to run grunt.

Rather than needing to install grunt globally, this allows running grunt from node_modules, instead.

npm run grunt ... is effectively the equivalent of running grunt ....

Props andrew.taylor, pento.
Merges [43836] to trunk.
See #45214.

Built from https://develop.svn.wordpress.org/trunk@44176


git-svn-id: https://core.svn.wordpress.org/trunk@44006 1a063a9b-81f0-0310-95a4-ce76da25c4cd