Build/Test Tools: Copy `build/routes` and `build/pages` JS files.

The JavaScript fils within the `build/routes` and `build/pages` directories within the built `gutenberg` asset were not being copied properly into the working directory.

This configures a `grunt copy:gutenberg-js` task to handle this.

Follow up to [61873], [61874].

Props jorgefilipecosta, ellatrix, desrosj.
See #64393.

git-svn-id: https://develop.svn.wordpress.org/trunk@61978 602fd350-edb4-49c9-b593-d223f7449a82