[css-forms] Specify layout behaviour for buttons

[lukewarlow]

This is obviously eventually something the HTML spec should control but while we're still working things out we should specify the expected layout behaviour for buttons.

Right now button elements and input type="button" have different layout behaviours which mean that the display: inline-flex style applied to input buttons doesn't actually work in layout.

https://html.spec.whatwg.org/multipage/rendering.html#button-layout-2