[Pivots] Option to use global custom elements registry

[nolanlawson]

Technically this can be done before or after pivots (#2724), but it's related.

We should have a flag that, when enabled, uses the raw customElements global registry to register components, rather than using a pivot system (or the quasi-pivot system we have today).

Benefits are:

1. Much less code can be shipped, assuming the flag is set at compile-time
2. With the flag, no need for global patching
3. APIs like customElements.get() and customElements.whenDefined() can be used to detect defined LWC elements, regardless of [Pivots] customElements.get/whenDefined should not return PivotConstructor #3073

[nolanlawson]

We may want to reverse the boolean logic on this flag and instead make it about enabling custom element registry pivots.

[nolanlawson]

Fixed by #3079