Partial hydration

Unsure if this should go here or in an RFC. I'll start here, but write a proper RFC if the scope is too big for a regular issue.

With the somewhat recent feature of disabling JS completely on pages it would be nice to selectively add sprinkles of JS around the page for some progressive enhancement.

Describe the solution you'd like
Svelte's action directive could work very well for this as. I briefly mentioned the idea to @pngwn and he mentioned that the life cycle hooks/methods wouldn't be usable (as we don't know when something is updated or destroyed. I think for a first version this would be fine - we would simply ignore the update and destroy functions. I don't this would be a big loss since you would lose the context of the page if you navigate away from it anyway, I think (or can you disabled JS but have the router still be enabled?).

Describe alternatives you've considered
An alternative way of doing this would be how Elder.js does it - a preprocessor that looks through your pages and finds any that are marked with some prop (sveltekit:partial?) and bundles that separately, injects that in some intersectionObserver solution. This would allow the usage of full components.

How important is this feature to you?
I can live without it, but partial hydration is 👌 and would give us a nice and easy way to enhance our static pages with whimsy and excitement! 👏

Start by reviewing Svelte's action directive and the existing behavior when JavaScript is disabled. Compare that approach with the proposed preprocessor and intersectionObserver alternative, including lifecycle limitations. Done should mean an agreed scope or RFC that defines the partial-hydration behavior and its constraints.