Upgrade build process

[bmuenzenmeyer]

Webpack would likely be the preferred method.

[sghoweri]

@bmuenzenmeyer perhaps have two separate tasks regarding the build and Webpack?

One regarding the build and config itself (ie this one) and the other (new issue) being the JavaScript module refactoring required to get things to be more in line with CommonJS or ES6 (ex. modules explicitly import their own required dependencies and export any exposed functions, etc)

Ex. regarding the JS modules themselves (to make things easier to upgrade or refactor): https://github.com/drupal-pattern-lab/styleguidekit-assets-default/tree/feature/modernize-patternlab-js/src/scripts

And adding Webpack to the build (configs in the root): https://github.com/drupal-pattern-lab/styleguidekit-assets-default/tree/feature/modernize-patternlab-js/build-tasks