[defer-hydration] Controlling hydration with the "defer-hydration" attribute

[justinfagnani]

This proposal allows us to control hydration and hydration ordering by server-rendering a defer-hydration attribute, and removing it on the client when we want to trigger hydration. This decouples element definition ordering from hydration ordering, and allows sections of the page to remain non-hydrated until an outside signal.

We have this implemented in the Lit SSR library.

I put an initial draft of the proposal in #15