Support exporting from a single figma file

[JonLz]

Thank you for the great library. This is a tool I am exploring using however my design team has a requirement to only maintain the color styles for both the light and dark color palette in a single figma file. I see that right now you only support a separate file for light and dark styles. Is this something you would entertain? I would be happy to help build out the feature if so.

I took an initial pass at it, and was able to prototype it out by modifying the ColorsLoader. If we add an export option, update the documentation, and configure the loader to be able to make this modification it should work. What do you think?

func load() throws -> (light: [Color], dark: [Color]?) {
        let colors = try loadColors(fileId: params.lightFileId)
        let darkSuffix = "_dark"
        let lightColors = colors
            .filter { !$0.name.hasSuffix(darkSuffix)}
        let darkColors = colors
            .filter { $0.name.hasSuffix(darkSuffix) }
            .map { color -> Color in
                var newColor = color
                newColor.name = String(color.name.dropLast(darkSuffix.count))
                return newColor
            }
        return (lightColors, darkColors)
    }

[subdan]

This is a great feature. I’ll be glad to accept your pull request.

What about dark illustrations? Do you want to store light and dark illustrations in a single Figma file with a "_dark" prefix?

[JonLz]

I will start with colors. We aren't planning to use the illustrations feature yet. My initial thought is it's probably more common to want to store colors in the same file, and less so for illustrations. If we hear more folks want it, it would be straightforward to implement it.

[subdan]

Thanks for the PR! I will check it withing 2 days.