Support multiple layers per include file

[keichan34]

Is your feature request related to a problem? Please describe.
Discussion: #114

When multiple layers are always related (for example, road lines + road casing or building fill and building outline), it can be cumbersome and error-prone to include multiple files for the same logical layer group.

Describe the solution you'd like
When importing layers, !!inc/file should be able to import multiple layers from one file.

Describe alternatives you've considered

• flatten the layers array after processing
• patch !!inc/file to flatten a root array when importing ← this is probably dangerous because when you're importing filters, you normally want to preserve the array structure, potentially leading to breaking changes
• create a new !!inc/layer function that flattens the root array

I think flattening the layers array is the best option for going forward with this? I don't expect the layers array to support nested arrays in future versions, and I don't think creating a new !!inc/layer function is warranted if it's just for this. If it supported something else that would be specific to importing layers, then maybe...

Additional context
n/a