fix: only throw bind error when not passing a value

[dummdidumm]

This is a proposal to relax the runtime error to only throw when you're passing a binding with an undefined value. This makes it possible to provide components in a way that can be used more flexibly while keeping the error to guard against the case that we want to avoid: a default value propagation up.

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Name	Status	Preview	Comments	Updated (UTC)
svelte-5-preview	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jan 9, 2024 4:07pm

[dummdidumm]

I'm wondering if we should instead make this a warning at DEV time only, and just not propagate the fallback value upwards. That way people could also bind to undefined, it just wouldn't be filled with the default value from the child.
If we did this, we could change the fallback behavior consistently for props to use the fallback value everytime undefined is passed, and values would get out of sync everytime the parent passes undefined and the child has a fallback value (regardless of being bound or not), not just the first time, which feels more consistent to me.