Allow button spacing to be configurable (or remove)

We have the following styles applied to adjacent buttons:

https://github.com/dequelabs/cauldron/blob/8bc06dcad8f0ac455107263b0f0bbf882e126afe/packages/styles/button.css#L250-L252

The problem with this approach, is that if buttons are placed inside of a flex/grid container with gaps, the margins will be inside of the gap created by styles. The only way to override this is by setting an overriding style which isn't ideal. We should consider allowing this to be controlled in the short term by allowing for the margins to be configurable OR don't apply margins at all and let authors decide how they want to space buttons.