Split client and server transporters

Describe the problem

The transport hook added in 2.11.0 is insufficient for returning long lived variables from load functions, such as ReadableStreams/async iterators and Observables

Describe the proposed solution

Split the transport encode/decode into hooks.server.js and hooks.client.js to allow for long lived variables to be serialized into IDs by the user, then revived on the client using an SSE or Websocket connection, similar to how Promises are already serialized into unique IDs that are attached to the window and resolved later.

Ideally this should also not leak the server implementation to the client

Alternatives considered

You can return an ID from a load function, then set up the SSE connection from the client, but this is brittle and not reusable

Importance

nice to have

Additional Information

Feature announcement: https://bsky.app/profile/svelte.dev/post/3ld4ncjryfs2g

Feature PR: https://github.com/sveltejs/kit/pull/13149
PR that would fix this: https://github.com/sveltejs/kit/pull/13125

Similar to: https://github.com/sveltejs/kit/issues/13156

Start by reviewing the transport encode/decode hooks described in the issue, along with hooks.server.js and hooks.client.js and the linked PRs 13149 and 13125. Done means server and client transporters are split so long-lived values can be serialized and revived without leaking the server implementation to the client.