Move all properties of EventTarget to symbols to avoid polluting the global scope (#48962)

Summary:
Pull Request resolved: #48962

Changelog: [internal]

We moved away from private fields for `EventTarget` for performance, in favor of normal fields prefixed with underscore, but this would pollute the global scope when we make it extend `EventTarget`.

This refactors the implementation to use symbols for all properties to avoid that problem, leaving only `addEventListener` and `removeEventListener` as regular properties in the prototype.

Performance-wise this is neutral or a slight improvement according to the existing benchmark.

Reviewed By: javache

Differential Revision: D68672215

fbshipit-source-id: b329548efce6059ae2b9f33afa0719e057d3b8ba

Author: rubennorte