feat: more efficient if block compiler output

[Rich-Harris]

Small tweak. Given this input...

{#if condition}
  <p>hello</p>
{/if}

...we currently generate this:

$.if(
  node,
  () => $.get(condition),
  ($$anchor) => {
    /* Init */
    var p = $.open($$anchor, frag_1);

    return $.close($$anchor, p);
  },
  null
);

With this PR:

$.if(node, () => $.get(condition), ($$anchor) => {
  /* Init */
  var p = $.open($$anchor, frag_1);

  return $.close($$anchor, p);
});

My eventual goal (will take some time to get there):

$.if(node, () => $.get(condition), ($$anchor) => $.append($$anchor, frag_1()));

Before submitting the PR, please make sure you do the following

• 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
• Prefix your PR title with feat:, fix:, chore:, or docs:.
• This message body should clearly illustrate what problems it solves.
• 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

[changeset-bot]

🦋 Changeset detected

Latest commit: 15a60ad

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
svelte	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR