Adds `materialGapSize` property to `ExpansionPanelList` widget.

| `materialGapSize: 0` | `materialGapSize: 16` (default) | `materialGapSize: 30` |
| --- | --- | --- |
| ![screenshot_1680450783](https://user-images.githubusercontent.com/13456345/229366131-8dd105cd-6371-4852-b57b-5900e3dbd203.png) | ![screenshot_1680453525](https://user-images.githubusercontent.com/13456345/229366640-e2ae5f95-0cd6-4694-b8ff-0de7fb7c8554.png) | ![screenshot_1680450910](https://user-images.githubusercontent.com/13456345/229366742-3121331b-b22e-46a1-bdfa-20b5663cbe2b.png) |
| ![screenshot_1680450785](https://user-images.githubusercontent.com/13456345/229366133-bf345914-9d74-4dcc-afc4-a6a9ccca13ab.png) | ![screenshot_1680453527](https://user-images.githubusercontent.com/13456345/229366644-bfff8e4f-8a66-4419-90b8-6a16442b7157.png) | ![screenshot_1680450912](https://user-images.githubusercontent.com/13456345/229366745-364b1a1e-ed2c-4463-a74d-d8e94bf509b9.png) |
| ![screenshot_1680450787](https://user-images.githubusercontent.com/13456345/229366138-3f6280f0-e58a-4621-aced-c7eee2077ef8.png) | ![screenshot_1680453530](https://user-images.githubusercontent.com/13456345/229366648-3805db23-29b0-46a3-82ce-92688e65f0fd.png) | ![screenshot_1680450914](https://user-images.githubusercontent.com/13456345/229366747-73d46f9c-3edc-4197-8f02-651ff8499323.png) |
| ![screenshot_1680450789](https://user-images.githubusercontent.com/13456345/229366140-4ff8572c-8d06-4985-a697-3af0ce0e5743.png) | ![screenshot_1680453532](https://user-images.githubusercontent.com/13456345/229366651-88af37ac-7503-4abc-b6a6-d8556a583871.png) | ![screenshot_1680450916](https://user-images.githubusercontent.com/13456345/229366749-0db156b0-6b53-4ef2-9699-0d3e8704433b.png) |

Fixes: #118167