feat: better generated each block code in SSR mode

[Rich-Harris]

This improves each block output in SSR mode in two ways — first, we get rid of an unnecessary declaration...

-let $$item = array[index];
-let { blah } = $$item;
+let { blah } = array[index];

...then we cache the length of the array before iterating over it:

-for (let $$index = 0; $$index < array.length; $$index += 1) {
+for (let $$index = 0, $$length = array.length; $$index < $$length; $$index += 1) {

We're also able to simplify the types — the $$item stuff now only exists during the client transform step, so we can remove it from the shared each.metadata object

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: b08f7ea

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

This PR includes changesets to release 1 package

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svelte	Patch

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