chore: update sequencing inside blocks

[Rich-Harris]

Right now, blocks are created after local updates have been applied. Given something like this...

<script>
  let number = $state(0);
</script>

<!-- 1 -->
<input type="number" bind:value={number} />

<!-- 2 -->
{#if number % 2 === 0}
  <p>{number} is even</p>
{:else}
  <p>{number} is odd</p>
{/if}

<!-- 3 -->
<p>{number} * 2 = {number * 2}</p>

...the compiler spits out something like this:

let number = $.source(0);
var fragment = $.open_frag($$anchor, frag);

/* 1 */
var input = $.first_child(fragment);
$.remove_input_attr_defaults(input);

/* 2 */
var node = $.sibling($.sibling(input, true));

/* 3 */
var p_2 = $.sibling($.sibling(node, true));
var text_2 = $.child(p_2);
$.render_effect(() => $.set_text(text_2, `${$.stringify($.get(number))} * 2 = ${$.stringify($.get(number) * 2)}`));

/* 1 */
$.bind_value(input, () => $.get(number), ($$value) => $.set(number, $$value));

/* 2 */
$.if(
  node,
  () => $.get(number) % 2 === 0,
  ($$anchor) => {
    var p = $.open($$anchor, frag_1);
    var text = $.child(p);

    $.render_effect(() => $.set_text(text, `${$.stringify($.get(number))} is even`));
    return $.close($$anchor, p);
  },
  ($$anchor) => {
    var p_1 = $.open($$anchor, frag_2);
    var text_1 = $.child(p_1);

    $.render_effect(() => $.set_text(text_1, `${$.stringify($.get(number))} is odd`));
    return $.close($$anchor, p_1);
  }
);

This is... fine, but it would perhaps be clearer if we got something like this instead:

/* 1 */
var input = $.first_child(fragment);
$.remove_input_attr_defaults(input);
$.bind_value(input, () => $.get(number), ($$value) => $.set(number, $$value));

/* 2 */
var node = $.sibling($.sibling(input, true));

$.if(
  node,
  () => $.get(number) % 2 === 0,
  ($$anchor) => {
    var p = $.open($$anchor, frag_1);
    var text = $.child(p);

    $.render_effect(() => $.set_text(text, `${$.stringify($.get(number))} is even`));
    return $.close($$anchor, p);
  },
  ($$anchor) => {
    var p_1 = $.open($$anchor, frag_2);
    var text_1 = $.child(p_1);

    $.render_effect(() => $.set_text(text_1, `${$.stringify($.get(number))} is odd`));
    return $.close($$anchor, p_1);
  }
);

/* 3 */
var p_2 = $.sibling($.sibling(node, true));
var text_2 = $.child(p_2);
$.render_effect(() => $.set_text(text_2, `${$.stringify($.get(number))} * 2 = ${$.stringify($.get(number) * 2)}`));

This PR doesn't produce exactly that order, since it turns out we need to apply actions/bindings/events after the final render_effect, otherwise things go awry. But it's very close to that ideal.

The reason this is valuable is that it should, if my hunch is correct, unlock a slightly simpler approach to hydration. (It might be worth waiting until we can actually test that theory before merging this, to avoid needless changes.)

Note that I had to compromise in one place: an action inside a block (such as an {#if ...} or {#each ...} will run before an action in an earlier element. In other words b will run before a:

<div use:a>...</div>

{#if foo}
  <div use:b>...</div>
{/if}

I don't consider that a big problem — actions ought to be independent of one another — but it's worth noting.

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• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
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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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