fix: make SvelteMap work with async mode

[Rich-Harris]

Pushing this up even though it has failing tests. The reason the new test fails is because we keep doing {await push(...)} every time a promise resolves, because the each block effect (which sets each item's .v source) re-runs on every traversal. This is because

• we always add block effects to the #maybe_dirty_effects set on traversal
• for some reason, is_dirty is returning true when it should be false

I intend to investigate both of these, because it seems like we're creating inefficiency at best, and incorrect behaviour at worst.

One thing I noticed is that over-execute deriveds when time-travelling, because effect_tracking() is false inside is_dirty:

svelte/packages/svelte/src/internal/client/reactivity/deriveds.js

Lines 403 to 407 in 58f161d

	// only cache the value if we're in a tracking context, otherwise we won't
	// clear the cache in `mark_reactions` when dependencies are updated
	if (effect_tracking() || current_batch?.is_fork) {
	batch_values.set(derived, value);
	}

If we could fix that, and get the invalidation right, then maybe we could preserve derived values across batch runs? e.g. we include them in batch.current, rather than just the ephemeral batch_values. Unfortunately I anticipate this being a deep rabbit hole

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