Allow optional chaining in JS library code

[sbc100]

This change verifies that closure can successfully lower the optional chaining away on older browsers.

This also requires bumping of chrome and safari minimum versions.. so maybe its not worth it?

[sbc100]

I'm not sure if the value we will get out of this is worth the min version bump.. but maybe those two versions are fine to bump anyway since they are both evergreen browsers?

@juj @dschuff @kripken ?

[kripken]

The version bumps don't trouble me much, yeah, they look minor. But do you have a sense for the possible benefit here? If this is a common pattern in our JS code that might be nice, but offhand I can't think of places that would benefit.

[sbc100]

Yeah, I'm struggling to see where would save a lot by using this.. its kind of hard to search for possible places where it might apply but I'll give it a go.

[juj]

LGTM, the browser version bumps look fine. Unity currently requires Firefox 78.9.0esr as minimum, and soon will bump to Firefox 100-something esr because older versions had a bug with multithreading.

I think it would make sense to group ?., ??, and ??= operators behind the same feature item. Looking at min versions, all other browsers added support in the same version, except for Firefox, where there was a two version gap. To accommodate for that, we can just take the newer Firefox 74 as the min version.

[sbc100]

Superseded by #20549