How to position javascript file as the last one to load?
I was surprised hear that "only negative values are allowed" as JS weights, so I had to hunt that down.  And it's true... https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Asset%21LibraryDiscoveryParser.php/function/LibraryDiscoveryParser%3A%3AbuildByExtension/8.2.x There is an exception error message that says, "The $extension/$id library defines a positive weight for '$source'. Only negative weights are allowed (but should be avoided). Instead of a positive weight, specify accurate dependencies for this library."

This of course reminded me that, yes indeed, dependencies were added/improved in Drupal 8 so that we didn't have to worry about this.  Oddly enough, there are several core JS files that define weight, so I guess we are only part way there.

So, this is just a long-winded way of saying, what scripts does the Zurb Foudation depend on?  And can you list those dependencies in your libraries file under the Zurb script?  See https://www.drupal.org/docs/8/creating-custom-modules/adding-stylesheets-css-and-javascript-js-to-a-drupal-8-module for more info.