feat: turn reactive_declaration_non_reactive_property into a runtime warning

[Rich-Harris]

Alternative to #14111. Instead of a compiler warning, we can have a runtime warning for cases where 'invisible' reactive state is read inside a $: statement. The message could probably be improved and it might be nice to have a detailed example for the docs, but putting this up just as a suggestion for now

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🦋 Changeset detected

Latest commit: 2207b7d

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[Rich-Harris]

preview: https://svelte-dev-git-preview-svelte-14192-svelte.vercel.app/

this is an automated message

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@14192

[dummdidumm]

As I commented on that other PR I think this approach makes more sense, so 👍

[benmccann]

I agree this approach makes sense