fix: properly set value property of custom elements

[dummdidumm]

Avoid going through the element.value = element.__value = newValue condition because __value is actually how Lit stores the current value on the element, and messing with that would break things: Lit would think the value hasn't changed (because __value is already set to the new value by us) and doesn't fire an update.

fixes #15194

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pnpm add https://pkg.pr.new/svelte@15206