fix: link offscreen items and last effect in each block correctly

[dummdidumm]

It's possible that due to how new elements are inserted into the array that effect.last is wrong. We need to ensure it is really the last item to keep items properly connected to the graph. In addition we link offscreen items after all onscreen items, to ensure they don't have wrong pointers.

#17244 as an alternative didn't work because it is possible due to how we don't link all items during reconciliation to end up with the wrong effect.last, which subsequently can mean that an item "falls out" of the graph. The added test showcases how this goes wrong with the previous logic:

1. start with [1, 2, 3]
2. create 4 -> is now effect.last
3. insert at second position -> [1, 4, 2, 3] -> effect.last now 2 (not 3 because that isn't relinked because the order didn't change and reconciliation sees that)
4. create 5 -> is now effect.last, which means 2.next is now 5 and no longer 3
5. insert at third position -> effect.last now 2 again -> noone has 3 as next -> 3 fell out of the graph
6. modify 3 -> nothing happens

The fix is to go back to the behavior prior to #17150 which is "set the last item as last", additionally taking into account offscreen items.

Fixes #17201
Fixes #17239

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Playground

pnpm add https://pkg.pr.new/svelte@17240

[Rich-Harris]

I think the fix should be in link rather than reconcile: #17244

[Rich-Harris]

While digging into this I built a stress test that checks whether reconciliation succeeds and effects remaining linked to the tree. It turns out that there are some tricky bugs around transitions that are unaffected by this PR; I aim to investigate those next.

In the meantime, it looks like this PR succeeds where main fails with transitions disabled (try preset 3 with this branch and 5.44.1. I would love it if we could avoid the extra list traversal, but for now I think it's a price worth paying.