Implementation of set.equals

[mvaligursky]

extracted from #2990

[willeastcott]

I can't find this function in the JavaScript standard library:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set

If Set#isEqual is not in the JS language spec, then this is not a polyfill.

[mvaligursky]

it's a polyfill from the future when JS implements this function ;)

do we have some better place to add these? Perhaps we could create a new folder for these. Or just add it to "/core"?

[willeastcott]

My feeling is that if we are not polyfilling a legitimately existing JS language feature/function, we shouldn't screw with standard library classes, poking in our own stuff. You could stick a utility function in core\set-utils.js or something? But yeah, operate on set instances rather than inject stuff into Set.

[Maksims]

Feels like it could be better named, consistent with Vec3 for example: equals.