feat: support for spreading function bindings

[jackgdll]

Closes: #16086

Adds support for spreading function bindings using ... syntax.

Before:

<script>
  const [get, set] = bindLowerCase(value);
</script>
<input bind:value={get, set} />

After:

<script>
  function bindLowerCase(value) {
    return [
      () => value.toLowerCase(),
      (v) => value = v.toLowerCase()
    ];
  }
</script>
<input bind:value={...bindLowerCase(value)} />

Works with arrays [get, set] and objects {get, set}.
If either get or set is nullish it's a noop.
If either get or set is not a function a runtime error is thrown.

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• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.
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Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint

[changeset-bot]

🦋 Changeset detected

Latest commit: 9c05042

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

This PR includes changesets to release 1 package

Name	Type
svelte	Minor

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-16607-svelte.vercel.app/

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@16607

[Rich-Harris]

Thank you! I think this would need to run inside an effect, or use a derived, since otherwise something like this...

<input bind:value={...binding(validation, defaults)} />

...wouldn't update if validation or defaults did. Or to take an example similar to the one above, toggling uppercase has no effect, and it feels like it probably ought to.

[jackgdll]

Thanks! I wrapped the call to $.validate_spread_bindings in a derived and added a test for this case