Adds new property `separatorColor` to `CupertinoListSection` widget.

Fixes: #124744

### `separatorColor` property is null

<img src="https://user-images.githubusercontent.com/13456345/231840875-22605c23-ee9f-464f-a005-c9fce1b641a7.png" width=350>

### `separatorColor` property is not-null

| Color.fromARGB(255, 136, 13, 207) | Color.fromARGB(255, 13, 62, 207) | Color.fromARGB(255, 29, 207, 13) |
| --- | --- | --- |
| ![a](https://user-images.githubusercontent.com/13456345/231840832-6027cf86-e9c0-4206-b6f8-23556471c1bd.png) | ![b](https://user-images.githubusercontent.com/13456345/231840862-dcc30212-14f8-44af-a8cb-d04b22b7a078.png) | ![c](https://user-images.githubusercontent.com/13456345/231840870-d9fafe23-e9a4-423a-a25c-b71835991d07.png) |