feat: Export createReactiveFunction

[elliott-with-the-longest-name-on-github]

TODO:

• Do we want to do this? (Seems like yes)
• Is JSDoc good?
• Tests
• Should we export ReactiveValue? IMO no, as this seems like the better way to do this, especially when combined with $derived.by, but I'm not completely attached to that opinion
• Changeset

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-15633-svelte.vercel.app/

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Playground

pnpm add https://pkg.pr.new/svelte@15633 

[dummdidumm]

Should we expose the underlying ReactiveValue class as well (or instead)?

[elliott-with-the-longest-name-on-github]

Yeah I called that out as well -- overall I think the function is more useful (it works with $derived.by like a charm), but it wouldn't hurt to have both...