feat: remove $.unwrap calls from each block indexes

[Rich-Harris]

See #12511 and #12530 for more context.

At present, we unnecessarily wrap each block indexes in unwrap:

{#each things as thing, i}
  <p>{i}: {thing.name}</p>
{/each}

$.each(node, 1, () => things, $.index, ($$anchor, thing, i) => {
  var p = root_1();
  var text = $.child(p);

  $.reset(p);
  $.template_effect(() => $.set_text(text, `${$.unwrap(i) ?? ""}: ${$.unwrap(thing).name ?? ""}`));
  $.append($$anchor, p);
});

We know that i is a number, here; $.unwrap(i) is unnecessary. In the case of a keyed block, we can use $.get(i). And in the rare case that a key function references the index, we can always just reference it (and the item) directly.

We shouldn't be using $.unwrap anyway because duck typing doesn't work (I guess we could faff around with a private symbol, but let's not) — this PR doesn't kill it off completely but it gets us part of the way there. Will work on that in a follow-up PR.

I didn't include a snapshot test, because when we remove unwrap from the codebase it'll no longer be necessary.

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• Ideally, include a test that fails without this PR but passes with it.

Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint

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Latest commit: abc87f4

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[Rich-Harris]

going to self-merge this to unblock further work