Make ARIA reflection polyfill optional

[nolanlawson]

See: #2733 (comment)

Steps:

1. Create a feature flag
2. When the flag is enabled, do not apply the ARIA reflection poyfill to the global Element.prototype
3. Instead, when the flag is enabled, apply it selectively only to LightningElement

This should be the minimum we can do to get rid of the global polyfill while not totally breaking backwards compatibility. It shouldn't require any template compiler changes, and the only thing it would break is:

const div = document.createElement('div')
div.ariaActiveDescendant = 'foo'
div.getAttribute('aria-activedescendant') // "foo" because reflected by the polyfill

On LightningElements, the ARIA property/attribute reflection would continue working like before.

However, since this is technically a breaking/observable change, all observable changes should be behind the flag.

[git2gus]

This issue has been linked to a new work item: W-12107800