add 'getGroupLayersId' method && fix bugs

[wang1212]

• add getGroupLayersId method

Although I can iterate over all the layers to get the ids of all the layers of a particular id group, I think it's best to hide the implementation details of metadata.group for the user, so I added the getGroupLayersId method.

• fix metadata be undefined exception

All layers added through the plugin have a non-empty metadata attribute, but I don't think there is any need to add all layers through the plugin, such as a map initialization layer, in which case the layer's metadata will be undefined, causing the exception to occur.

[wang1212]

I also found that the addLayerToGroup(map, groupId, layer, beforeId) method gets the wrong beforeId when beforeId is not specified.

• source code

if (beforeId && !ignoreBeforeIdCheck && (!isLayer(map, beforeId) || getLayerGroup(map, beforeId) !== groupId)) {
	throw new Error('beforeId must be the id of a layer within the same group');
} else if (!beforeId && !ignoreBeforeIdCheck) {
	beforeId = getLayerIdFromIndex(map, getGroupFirstLayerId(map, groupId) - 1);
}

• modified code

if (beforeId && !ignoreBeforeIdCheck && (!isLayer(map, beforeId) || getLayerGroup(map, beforeId) !== groupId)) {
	throw new Error('beforeId must be the id of a layer within the same group')
} else if (!beforeId && !ignoreBeforeIdCheck) {

	const lastLayerIndex = getGroupLastLayerIndex(map, groupId)

	if (lastLayerIndex === -1) {
		throw new Error(`the "${ groupId }" layer group does not exist`)
	}

	beforeId = getLayerIdFromIndex(map, lastLayerIndex + 1)
}