Define code style to make accepting contributions easier

As a contributor, I'd like to make it as easy as possible to review and have my contributions approved. In making that possible, I'd like to propose adding code style checks as a part of the build.

I'm happy to adopt the style that already exists in the repository, so all I'm asking here is whether such a change would be of interest if I do all of the work.

Start by reviewing the repository's existing JavaScript style and build entry points, then determine where a style check could run. Done means the style check is agreed on and integrated into the build so contributions are checked consistently.

Written by the indexing model from the issue text.