Properly merge incoming props with own props

[RobinMalfait]

This PR will apply the same logic as we did in the React version where incoming props should be
merged with our own props.

• If we internally provide an onClick, then it should first call the onClick of the incoming prop then our own onClick.
• If the incoming onClick runs event.preventDefault() then the rest of the onClick listeners (e.g.: our internal onClick) will not be fired.

Fixes #1579

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Name	Status	Preview	Updated
headlessui-react	✅ Ready (Inspect)	Visit Preview	Jul 7, 2022 at 2:47PM (UTC)
headlessui-vue	✅ Ready (Inspect)	Visit Preview	Jul 7, 2022 at 2:47PM (UTC)