Allow ScrollProp to be optional

[gordanielyan]

Inertia::scroll() is great for implementing infinite scrolling, but today it always loads as part of the initial response (or immediately after, when deferred).
This makes it awkward to use for shared data (e.g. notification list) that:

• should not always be loaded
• should only be fetched when actually needed
• should integrate naturally with WhenVisible

This PR adds an optional() modifier:

Inertia::scroll(fn () => User::paginate())->optional();