Groups a set of tile map layers together, allowing them to share a provided [LayeredTileSet].
Groups together tile map layers as part or the same map, replacing the [LayeredTileMap] node. Child layers will use this node's [member tile_set].
The editor also uses [LayeredTileMapLayerGroup] as a way to store which layers are selected in a given group. This allows highlighting the currently selected layers.
The assigned [LayeredTileSet]. This LayeredTileSet will be applied to all child layers.