General housekeeping

In this change we're just getting on top of some basic things we've either ommitted when making changes or because we have made a concious decision about a way forward.

- Using kebab-case rather than snake_case for filenames
  - Though there is no standard or rule, it's our opinion that most javascript projects use kebab-case
- Ensuring all require statements include the extension
  - We know it's not needed. But as [Switch from CommonJS to ES6 modules](#9) shows, we really want to switch to imports one day and this will set us up for that
- Including module JSDoc comments at the top of each file
  - We use JSDoc to comment our methods. I also reccommends CommonJS modules should include a standalone comment. When you do, VSCode will display the info when you hover over a `require()`