[Blazor] Prevent unwanted DOM merging during enhanced page updates

There might be times when an enhanced page update results in DOM elements getting merged/recycled in an undesirable manner. This is fine in many cases but can sometimes cause strange side effects (e.g., weird CSS animations playing on merged elements).

We should consider introducing a `data-key` attribute that can be used to manually indicate that a DOM element can only be merged with another element with a matching `data-key` attribute.

A current workaround is to wrap content in an element with a unique tag name to prevent it from getting merged. This could be an unresolved custom element tag name, e.g., ``.