Splitting source files

Plugin and (relative) JS/TS n00b here…

Starting from the boilerplate generated by Plugins > Development > New Plugin, which expects all your code to be in the code.ts and ui.html files, what’s the easiest way(s) to add additional .js, .ts, and .css files to the project? (At the moment, I’m mainly interested in pulling a bunch of JS out of ui.html, so that it really is mostly HTML.)