Blockless redux

[Rich-Harris]

This marks the start of another run at #10594 — trying to do it in a more incremental fashion this time. This does mean that things may look suboptimal initially.

Right now, block effects and their branch effects (still workshopping these names) are created as siblings. Because of that, we have to do things like this:

if (type === IF_BLOCK) {
  // a bunch of special logic, far from the if block definition
}

If we just construct the tree correctly in the first place, that becomes unnecessary. Eventually, the block construct itself (as distinct from the block effect) also becomes unnecessary, and we can greatly simplify things like transitions.

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