fix: create offscreen effects with correct anchor

[Rich-Harris]

This is another take on #18079. The symbol thing felt like a bit of a kludge, so I started poking about at possible alternatives and this is where I landed.

Having done so I think maybe @dummdidumm was right the first time (many such cases) — without the symbol, the only way to get the HMR wrapper to send the right anchor to its component is to create the effect onscreen and then remove it if we want to defer it, which is extra work. Still, there might be some merit in only storing the offscreen effects rather than fragments — not sure — so I figured I'd create a PR for posterity.

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Preview: https://svelte-dev-git-preview-svelte-18086-svelte.vercel.app/

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Playground

pnpm add https://pkg.pr.new/svelte@18086

[dummdidumm]

Interesting approach!
I guess another approach to fix the issue would be to not just pass the anchor along when creating a dynamic component, and instead create a intermediate comment (like it's done for if blocks) which the HMR wrapper then prepends to. We could make it dev-only to not pollute prod with even more comments than we already do (conversely there's probably a lot of potential of getting rid of some unnecessary comments in prod by detecting sole children; but that's a different task)