fix: ensure $effect.root is ignored on the server

[dummdidumm]

Alternate PR to #12331 - choose one.

Ignore the contents of the effect root, just return a noop where necessary

fixes #12322

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[Rich-Harris]

I'm inclined towards this fix over the other one, but will wait to see if others feel strongly

[dummdidumm]

I'm mostly indifferent. I think it doesn't matter too much either way, because for this to have any practical consequences you would have to do some kind of eager side effect inside the effect root (like logging, which you want to do on the server, too), or jump through hoops to create a variable outside the effect root, eagerly assign to it from inside, then return that variable synchronously, and do something with the result (like using it as an attribute value).

I think it mainly comes down to whether or not it's easily possible to move the stuff you want to run in SSR outside of the effect root, so it always executes.

[abdel-17]

I think a weird edge case with running $effect.root only on the server is manual cleanup.

Say I have some utility that creates effects under the hood. If I want it to be global, I have to initialize it inside of an effect root.

let foo: Foo;

const cleanup = effect.root(() => {
  foo = new Foo();
});

export { foo, cleanup };

[dummdidumm]

We decided to go with this variant for now, for the following reasons:

• the other $effect runes are also ignored on the server
• it's an edge case (and should remain one) to use this rune at all
• it's an even bigger edge case to use these on the server in a way that they are used eagerly while rendering