[material_ui] Add ReorderableListView.separated constructor

[krolmic]

Ports the Material half of flutter/flutter#190131 per flutter/flutter#188444: a reorderable list with separators between items, mirroring ListView.separated.

This ports the Material half of flutter/flutter#190131 to material_ui,
following flutter/flutter#188444 (Material is frozen in
flutter/flutter). The widgets half (SliverReorderableList.separated,
which this constructor wraps) remains in flutter/flutter#190131. The API
example wraps each ListTile in its own keyed Material so tile
backgrounds move with their tiles during a drag, addressing
review feedback
from @Piinks on the original PR (tileColor is painted by the nearest
ancestor Material, per the ListTile documentation).

This PR depends on SliverReorderableList.separated landing in
flutter/flutter (flutter/flutter#190131). Once that API is in a Flutter
release CI can use, I'll bump flutter: in pubspec.yaml and mark
this ready for review.

Fixes flutter/flutter#76706

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[gemini-code-assist]

Code Review

This pull request introduces the ReorderableListView.separated constructor to ReorderableListView, allowing the creation of reorderable lists with separators between items. It includes an example implementation, comprehensive unit tests, and a pending changelog entry. A critical syntax error was identified in the example file where EdgeInsets was omitted from the padding property.

[gemini-code-assist]

There is a syntax error here: .symmetric is missing the EdgeInsets class name. It should be EdgeInsets.symmetric.

Suggested change

	padding: const .symmetric(horizontal: 40.0),
	padding: const EdgeInsets.symmetric(horizontal: 40.0),