MutationObservable and IntersectionObservable methods are not enumerable

[OlaviSau]

Describe the bug
The methods of MutationObservable and IntersectionObservable are enumerable in browser implementations. This is used by zone.js to forward calls from the proxies created by zone.js

To Reproduce
Steps to reproduce the behavior:
MutationObserver.prototype.propertyIsEnumerable("observe") // => false

Expected behavior
MutationObserver.prototype.propertyIsEnumerable("observe") // => true

Device:
ALL

Additional context
Although I personally do not like this, I think those properties should be marked as enumerable. This can either be done by creating an utility decorator or by setting the property after the class has been created. This works in user land too, just have to set the values as enumerable before zone.js is ran.

Object.defineProperty(
	window.IntersectionObserver.prototype,
	"observe",
	{ enumerable: true, value: window.IntersectionObserver.prototype.observe },
);

[OlaviSau]

@capricorn86 LMK if you want me to create PR + tests for this.