Pass loading state as property to Deferred children

[scajal]

This PR updates the Deferred component so it will pass the current loading state to the children, when children is a function. It also makes the fallback property optional on the component.

This allows the developer to, for example, show a skeleton while the deferred props are being loaded.

Here's a react example:

<Deferred data="users">
{({ loading }) => (
    <form>
        <select disabled={loading}>
            {users?.map(user => <option key={user.id} value={user.id}>{user.name}</option>)}
        </select>
    </form>
)}
</Deferred>

[pascalbaljet]

I'm fine with passing the loading state to the children/slot, but I think making the fallback optional and removing the error is a breaking change. We could make that change in a next major release, though. Could you update the PR to just expose the loading state?

[scajal]

@pascalbaljet With a required fallback this wouldn't make any sense. The fallback would always be rendered if present and the data is still not loaded, so the children would only be rendered after loading the data and the loading property would always be true.