chore: move DOM-related effect properties to effect.nodes

[Rich-Harris]

Currently, an Effect has three properties that default to null — nodes_start, nodes_end, and transitions — and which are only used if the effect is a branch effect (or html/head effect, but you get the idea).

Since smaller objects are faster to work with and take up less memory, it's almost certainly preferable to reduce that to a single property, nodes, which is populated if it's the sort of effect that contains DOM.

My real motivation is this, though: I'm working through the edge cases in #17258 and it's hellishly complicated, made doubly so by the fact that every each block has two linked lists that resemble each other but don't fully match — a list of 'items' and a list of their effects. We only really need the second of these. Getting rid of the items list will reduce memory usage, and reduce the amount of work that needs to happen during reconciliation. But to get there, we need to move the AnimationManager from EachItem and onto Effect. If it became a direct property of Effect that would add unnecessary overhead (animations are exceedingly rare, relative to effects as a whole). But if it's part of nodes, it's fine. Hence the larger change.

(It's possible that by putting animations on effects, we could eventually relax the restriction that says the animate: directive has to be a direct child of a keyed each block, though that's a separate conversation that will probably be overtaken by a broader rethink of how animations and transitions work.)

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pnpm add https://pkg.pr.new/svelte@17293

[Rich-Harris]

gonna go ahead and self-merge this to unblock further work, otherwise it will become very hairy to juggle everything

[JonathonRP]

I just wanted to up vote the rethinking of animations, and I do like the idea of lifting animations restriction to just each block. I think a lot of us in animations have been wanting it but not noisy about it 😅