chore: simplify assignments in server code

[Rich-Harris]

Another thing I found during the recent refactor — the server version of serialize_set_binding contains some useless code. This tidies it up, colocates it with AssignmentExpression (the only visitor that uses it), and as a bonus improves the code generation along the lines suggested by #11921. Given this input...

[$a, $b] = values;

...we generate this:

-(() => {
-  const tmp = values;
-
-  (
-    $.store_set(a, tmp[0]),
-    $.store_set(b, tmp[1])
-  );
-
-  return tmp;
-})();
+(
+  $.store_set(a, values[0]),
+  $.store_set(b, values[1])
+);

It'd be good to do the same for client-side code, and we can probably reuse some code. I didn't want to get carried away though.

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: df19c83

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

[Rich-Harris]

Just found a bug in the existing code — we don't respect the operator when reassigning store values:

// in
$count += 1;
$count += 1;
$count += 1;

// out
$.store_set(count, 1);
$.store_set(count, 1);
$.store_set(count, 1);

Will fix in this PR.

[Rich-Harris]

Another existing bug: we don't handle await correctly. Edit: I don't want to disappear down that rabbit hole right now. It's better in this PR anyway because we need to create IIFEs in fewer scenarios