Add class example

[pearmini]

@ksen0 Hi Kit, how's everything going? As I mentioned before, I’m working on a data storytelling library for p5.js called p5.viz.js. After our discussion, I want to implement something like this:

let barY;

function setup() {
  barY = new p5.BarY();
}

However, I noticed that there isn’t an example in index.js showing how to bind a class to the p5 namespace, so I looked through the p5.js 2.0 source code and figured out how it works. I thought it might be helpful to add an example in index.js for future reference — that’s what this PR is for. Please let me know if this is the right approach.

Also, if you have a moment, I’d really appreciate it if you could take a look at this PR charming-art/p5.viz.js#1, which adds a bar chart to p5.viz.js. I’d love your feedback on whether the API design aligns with the p5.js style, and any other suggestions you might have!