chore: simplify transitions

[Rich-Harris]

unfortunately i don't think there's a more incremental path to #10594 than this — trying to overhaul all the transition code and then worrying about blocks, rather than trying to do both at once. excuse the dust

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Latest commit: b98b6c7

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[trueadm]

Here are some REPL examples that don't work quite right in this PR:

• Flip on simple list
• https://svelte.dev/repl/3846554f13114fcd838271085773edfb?version=4.2.9

Also this also seems slightly different too:

• https://svelte.dev/repl/6e891305e9644a7ca7065fa95c79d2d2?version=4.2.9