Devoncarew format css

Add a css formatter to spark. It's not on our critical path, but I was writing an app with Spark this weekend, and wanted this feature. We'll need to work on how to expose it to the UI in the future. Currently it's a keybinding; we'll want a better keybinding, and some button/menu-item in the UI to help user's discover the feature.

@dinhviethoa