docs: How to add support for ES6 syntax

[rubydusa]

Maybe I'm just being stupid, but it seems that simply installing the latest version of paper.js through the downloads section or through npm results in a version of paper.js that doesn't support es6 syntax.

As far as I could tell, the official solution is to load a version of acorn.js that does support es6 and then paper.js will prefer the loaded version when parsing paperscript.

Issues for reference:
paperjs/paper.js#1183
paperjs/paper.js#1357

This PR explains in the getting started tutorial how to overcome the issue.

[lehni]

Yes this is a bit of a legacy reason, in 2023 it doesn't make sense to include such an old version of acorn.js, we should simply not ship with it at all, because bundling the new version would make the library much larger.