When a Sliver with items is outside of the Viewport, but within the Viewport's `cacheExtent`, the framework should create SemanticNodes for the items even though they are out of view. However, for this to work, the Sliver's geometry must have a `cacheExtent` (how much space the sliver took up of the Viewport's `cacheExtent`) greater than 0, otherwise it is [excluded](f01ce9f4cb/packages/flutter/lib/src/rendering/viewport.dart (L311-L315)).
`SliverFillRemaining` widgets that fall outside the viewport did not have this set and therefore were being excluded when SemanticNodes were created, even if they were within the Viewport's `cacheExtent`. This PR sets the `cacheExtent` for `SliverFillRemaining` widgets.
In addition, `RenderSliverFillRemainingWithScrollable` would get dropped from the semantic tree because it's child had a size of 0 when outside the remaining paint extent. To fix, we give the child a `maxExtent` of the sliver's `cacheExtent` if it's outside the remaining paint extent but within the viewport's cacheExtent.
Fixes https://github.com/flutter/flutter/issues/142065.
Definitions:
* `RenderViewport.cacheExtent`:
```dart
/// The viewport has an area before and after the visible area to cache items
/// that are about to become visible when the user scrolls.
///
/// Items that fall in this cache area are laid out even though they are not
/// (yet) visible on screen. The [cacheExtent] describes how many pixels
/// the cache area extends before the leading edge and after the trailing edge
/// of the viewport.
///
/// The total extent, which the viewport will try to cover with children, is
/// [cacheExtent] before the leading edge + extent of the main axis +
/// [cacheExtent] after the trailing edge.
///
/// The cache area is also used to implement implicit accessibility scrolling
/// on iOS: When the accessibility focus moves from an item in the visible
/// viewport to an invisible item in the cache area, the framework will bring
/// that item into view with an (implicit) scroll action.
```
* `SliverGeometry.cacheExtent`:
```dart
/// How many pixels the sliver has consumed in the
/// [SliverConstraints.remainingCacheExtent].
```
* `SliverContraints.remainingCacheExtent`:
```dart
/// Describes how much content the sliver should provide starting from the
/// [cacheOrigin].
///
/// Not all content in the [remainingCacheExtent] will be visible as some
/// of it might fall into the cache area of the viewport.
///
/// Each sliver should start laying out content at the [cacheOrigin] and
/// try to provide as much content as the [remainingCacheExtent] allows.
```
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