State of polyfills and browser support

[mrxz]

Description:
There are several polyfills present in A-Frame, many of which are already quite dated. Several of them can probably be removed, but it mostly depends on the relevancy of the (old) browsers/devices that are still intended to be supported. AFAIK there is no set browserlist, but that might be a good idea going forwards.

Here's an overview of the polyfills:

Polyfill	API(s)	Caniuse %	Notes
@ungap/custom-elements	Custom Elements	78.83% + 18.5% = 97.33%	only for IE (Safari doesn't support customized built-in elements, but A-Frame does not require this)
custom-event-polyfill	CustomEvent	98.05% + 0.49% = 98.54%	only for IE 9+
object-assign	Object.assign	96.61%	ponyfill, only for IE
present	Performance.now	97.22%	⚠️ Also polyfilled by rStats.js
promise-polyfill	Promise	97.89%	only for IE
webvr-polyfill	WebVR	2.92% + 2.49% = 5.41%	contrary to what caniuse reports, FF has it disabled by default and requires specific flag to enable

Three.js has already dropped support for IE 11, so simply following suit, virtually all above mentioned polyfills can be removed as well. If a user does want to go through the pain of targetting IE 11, they can always load the needed polyfills themselves.

As for WebVR (polyfill), it would be nice to be able to get rid of it. There's quite a bit of duplicate code logic/paths due to supporting both WebVR and WebXR. I wouldn't be surprised if WebVR support is (subtly) broken in places.

Thoughts?