Add option for including JavaScript tests when scaffolding theme tests.

[joemcgill]

Currently, wp scaffold theme-tests sets up a PHPUnit framework for themes. Additionally, it would be great if there were an options for setting up JS tests.

Background: As part of WCUS 2018 contributor day, a team of us started working on creating a standardized setup for themes to use to add accessibility testing to their projects. You can see a proof of concept example of a setup, on @ryelle's fork of Twenty Nineteen, using Jest for the test suite and axe-puppeteer to run a11y checks inside assertions.

Once a lightweight scaffold is nailed down, it would be nice to add an option for theme devs to easily apply these best practices to their themes using the scaffold theme-tests command.