chore: follow-up to #11197

[Rich-Harris]

couple of small follow-ups to #11197:

• in push_template_node, effect is almost always the current_effect. we can use a default value and make the callsites more concise
• the if (current_dom[0].nodeType === 8) stuff makes me very nervous. feels like the sort of thing that could break easily, e.g. if preserveComments was enabled. I've felt for a while that we should probably just have a field on EachItem containing the opening anchor. This PR adds one. It means that an additional empty text node is created per each item (except during hydration, when the existing comments can be used), but it feels a bit more robust to me and avoids all the get_first_child calls

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