chore: more hydration stuff

[Rich-Harris]

more small tweaks:

• child_frag is a rather confusing name. changed it to first_child
• inside open_template, hydrate_nodes is always populated while hydrating — means we can delete some code
• replaced the slightly opaque ternary in close_template with a documented if block
• replaced Array.from(childNodes) with [...childNodes], after benchmarking it

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