Add CSS prop to nodes

[braposo]

After talking to @okonet one of his suggestions was to have a css prop in the query that would return the parsed css styles for each node.

I think this is a great suggestion and would be extremely useful for building components using figma-graphql.

We need to see which props can be exported as CSS but should be fairly easy to convert them to css props.

[braposo]

This might be useful to convert all properties to CSS https://github.com/figma/figma-api-demo/blob/master/figma-to-react/lib/figma.js