Set up JavaScript

[jasonaowen]

So far, the project hasn't used any JavaScript, but it's extremely likely to be needed sooner or later. Set up the project so that we can write JavaScript which is automatically built when the project is built (so that we can maintain a single-command build), but so that we use modern JavaScript build tools.

Previously, I've successfully used Node, the Gradle plugin for executing node scripts.

Consider setting up Stimulus to start out with, as it is "designed to augment your HTML with just enough behavior to make it shine."