SlotFill: remove registration API from useSlot result

[jsnajdr]

Follow-up to #67000 where @youknowriad pointed out in review that the API methods (registerSlot etc) returned from __experimentalUseSlot are not used anywhere. This PR removes the API methods and leaves only the slot object.

I also update the Fill component to not use useSlot at all: the register/unregisterFill can be accessed through context and to find out if there is a registered slot and whether it has a DOM element, we can look at registry.slots directly.

I was also searching for usages of __experimentalUseSlot. And it seems to be not really used by plugins. Plugins like WooCommerce or Kubio want to know whether a particular slot has registered fills. If there are no fills, they usually don't render the slot at all. They use __experimentalUseSlotFills( name ).fills.length > 0 to determine that. The __experimentalUseSlot function is called only for backward compatibility, because before #44642 there was only __experimentalUseSlot and it returned also the fills.

The only place where __experimentalUseSlot is really used is the Popover component, to determine whether a slot with given name is registered.

If we wanted to stabilize the useSlot API and remove the __experimental prefix, we need to solve these two use cases:

1. Find out whether a slot (by name) has any registered fills. This is widely used to render slots conditionally, only if there are fills registered. Because even an empty slot renders an element and is visible in the UI, and that's often not wanted.
2. Find out whether a slot (by name) is registered at all.

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Co-authored-by: jsnajdr <jsnajdr@git.wordpress.org>
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[jsnajdr]

Returning { ...slot } instead of slot has two purposes:

1. The hook always returns an object, even if the slot is undefined. Important for compatibility.
2. The hook has a nice return type: an object with optional ref and fillProps fields.

[youknowriad]

Makes sense.

Thanks for checking the usage in the directory.