fix: @attach opt's you into runes mode

[paoloricciuti]

Alternative to #15948 that opts you in runes mode whenever you use an attachment.

Personally I'm in more in favour of #15948 because:

1. this is technically a breaking change (although we could just label it as a fix since it was just released)
2. using attachments in legacy mode in not generally a problem, is just confusing when you do from an object and mutate another property of that object in the same component and a warning can already guide you through the right solution
3. this would mean that if you want to use the new feature of spreading attachments through component you would have to migrate your component which sometimes (for big components) might not be feasible.

But we could also do this.

Not sure which kind of test i should add for this, any ideas?

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[changeset-bot]

🦋 Changeset detected

Latest commit: ccdf3a7

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

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-15953-svelte.vercel.app/

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@15953

[Rich-Harris]

Alternative: #15962

[Rich-Harris]

closing in favour of #15962