fix: allow keyed {#each ...} without as

[Ocean-OS]

Closes #16927.
We had assumed that all keyed {#each ...} blocks would have an as clause, and didn't throw if this assumption failed, nor did we document this. This fixes that.

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Playground

pnpm add https://pkg.pr.new/svelte@16963

[Rich-Harris]

This wasn't quite right — the pattern argument always needs to be present, even if it's just placeholder, otherwise the index argument is given in the wrong position.

Fixed that but in the course of adding a test realised that there's actually a very good reason not to do this — if the key function can't reference the item, then it can't do anything useful. So I think we should probably just forbid key functions for each blocks without an as clause

[Rich-Harris]

opened #16966

[paoloricciuti]

Closing because of #16966