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.
See #45214.



git-svn-id: https://develop.svn.wordpress.org/branches/5.0@43836 602fd350-edb4-49c9-b593-d223f7449a82

package.json

@@ -92,5 +92,8 @@
 		"react": "^16.5.2",
 		"react-dom": "^16.5.2",
 		"whatwg-fetch": "^3.0.0"
+	},
+	"scripts":{
+		"grunt": "grunt"
 	}
 }