Adds a new `utils` bucket that's passed as a second arg when using a closure for theme values. The idea is you can destructure useful helper functions out of this argument, in this case a `negative` function that converts a positive scale to negative values. That's the only helper function right now, but making it a destructurable arg so we can add more if necessary without adding a bunch of positional arguments.
6.9.0 is EOL next month and our defaultConfig stub already uses features not supported in 6.x, which means I either had to change that file to use Object.assign instead of spread, or bump our dependency.
Would rather not have to write prehistoric JS just to support an almost-EOL version.
