When using items { label: string, value: string} input with value should only have value as value for native form support

[m-dressler]

Currently when using items with the Array<Object> variant, the value of the hidden input which receives the name attribute is set to JSON.stringify version of the Array<Object>.

Instead, when there is a key value in Object, the input should use that for its value such that no form transformation on the FormData is required.

[samal-rasmussen]

I believe this is the intended workaround for this:
#579
#581
https://svelte-select-examples.vercel.app/examples/slots/input-hidden

But JSON.stringify still causes issues for values that cannot serialize with JSON.stringify:
#700