Build legacy script for use as polyfill that hydrates shadow roots on page

Task to build a legacy script for use as polyfill that hydrates shadow roots on page, without needing to import modules and call `hydrateShadowRoots` in user code.

We may want one or more options on how this would work:
* Use `MutationObserver` to hydrate shadow roots as they are encountered, for better streaming support (as noted in the `mutation-observer.ts` implementation, this likely needs to be more sophisticated to avoid hydrating partially-parsed templates
* To confirm: Does the parser actually yield in the middle of a large template? If so, may need more sophisticated logic around e.g. detecting that the next element after the `` has opened before hydrating a given template
* What is the perf impact of using `MutationObserver` over one-shot `DOMContentLoaded`?
* Wait until `DOMContentLoaded` and upgrade all in one shot