feat: allow const tag inside svelte:boundary

[paoloricciuti]

Closes #14987

I don't see why we shouldn't allow const here but if you have any reason we can close. I left a TODO in the text because i noticed that reactivity is lost when the failed snippet is rendered but that's true in general (gonna open an issue about it in a sec).

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[changeset-bot]

🦋 Changeset detected

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[Rich-Harris]

preview: https://svelte-dev-git-preview-svelte-14993-svelte.vercel.app/

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[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@14993

[paoloricciuti]

I've added the test for when #14999 will be merged (for the moment it's failing cause it requires that PR)

[trueadm]

Do we need to update and docs for this change?

[paoloricciuti]

Do we need to update and docs for this change?

Uh right let me check