Some renderers, such as Lit, need special polyfills only for hydration. We have the `polyfills` array, but that is intended for polyfills that always need to run. This adds a second type hydrationPolyfills that only run on elements that are `:load`, `:idle`, etc.
* Implements low-level custom element support
* Changes based on self review
* Adds a changeset
* Polyfills are added even when not hydrating
* Remove hydrationMethod option
Punting on this idea until it's really needed.