Small fixes to help get dev-2.0 branch running for other contributors

[lukeplowden]

Changes:

• Add screenshots generated by npm run test to gitignore
• Change script reference in the 'empty example' folder to match rollup build
• Add temporary getters for properties absent on the color refactor which are referenced in the WebGL renderer API: p5.Color.levels, p5.Color._array
• Create the docs reference folder in case it doesn't exist

PR Checklist

• npm run lint passes

[limzykenneth]

The .gitignore and utils/convert.js changes are likely redundant with the merge of #7203 so they may not be needed anymore. The p5.Color.js one I'm not sure if those two values are being used somewhere still though? The API can change here so not necessarily need to retain these if they are no longer being used. (Edit: looks like its being used in WebGL, may need to review at a later point then)

The empty example folder I'm not sure it needs to have the script change to the rollup one just yet. If the idea is to have a dev environment going, npm run dev will use the one in preview/ although it is using module instance mode. I can set up one for the bundled global mode if that is the use case here.

[lukeplowden]

Thanks that all makes sense. I see your edit and yes they're still used in WebGL, the idea was just to put these temporary getters in now so WebGL can still be worked on in other areas. But also happy to just leave this as a local fix.

Dev environment was the use case so you can close this if you'd like

[davepagurek]

Right, I think the color changes are things we can take out once we do a bigger refactor of the webgl module, but this unblocks development for now.

As an aside, I think it's something we may want to include in our 1.x compatibility layer -- I've seen people accessing the color levels/array properties as a quick way to convert colors into shader uniforms. Maybe I should make an issue to start keeping track of those things? (It might also be worth giving those a "real" API with a better name, like toNormalizedRGB() or something, since passing a color into a shader isn't too uncommon.)

Edit: added #7230