Thanks for the detailed info. Have you already worked with browserify before? The idea is that you add all the dependencies in package.json (this case p5.js - note that the npm module is called “p5”). Bundling the sketch without p5 might work but not the way how it is intended.
I imported your project and made it working here - check package.json and app/sketch.js
The problem is that it seems p5.js as a module does not support global mode (more here). When I copied the instance-mode code from the link it works fine. I’m not sure if there is a way to make global mode here, but generally I would recommend switching to instance mode if you are using npm/browserify because weird things can happen around the namespace.