fix: ensure props passed to components via mount are updateable

[dummdidumm]

The detection whether or not props are writable is buggy; it doesn't take into account the case when instantiating components via mount or legacy-new

Fixes #14161

This posed an interesting question: What to do about (non-)bind:able properties? The answer I arrived on was: Treat it as if everything that can be bound is treated as bound, and everything else as readonly. In other words, if you're reassigning a prop, it will diverge from the passed in props if it's not bindable or not set in the parent, otherwise it will mutate the passed in props. I think that makes the most sense, given that you can't control this behavior from the outside.

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Latest commit: 06c5023

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Playground

pnpm add https://pkg.pr.new/svelte@14210

[Rich-Harris]

preview: https://svelte-dev-git-preview-svelte-14210-svelte.vercel.app/

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