Add `corner-shape` utility classes (`corner-*`)

[baptisthecht]

This PR adds utilities to control the shape of rounded corners of an element. These map to the new corner-shape property in CSS.

You can use the corner-* utilities to control the shape of all corners simultaneously or use utilities like corner-t-*, corner-bl-*, etc… to control individual corners.

Utility	Affected corners
corner-*	all
corner-t-*	top left, top right
corner-b-*	bottom left, bottom right
corner-l-*	top left, bottom left
corner-r-*	top right, bottom right
corner-tl-*	top left
corner-tr-*	top right
corner-bl-*	bottom left
corner-br-*	bottom right
corner-s-*	(logical) start start, end start
corner-e-*	(logical) start end, end end
corner-ss-*	(logical) start start
corner-se-*	(logical) start end
corner-ee-*	(logical) end end
corner-es-*	(logical) end start

Each utility has a handful of pre-defined values that map to their counterparts in CSS. Shown here are the versions for shaping all corners but they're present individual corner utilities as well:

Utility	Value
corner-round	corner-shape: round
corner-scoop	corner-shape: scoop
corner-bevel	corner-shape: bevel
corner-notch	corner-shape: notch
corner-square	corner-shape: square
corner-squircle	corner-shape: squircle

Additionally, arbitrary values are supported so you can use custom superellipse(…) values e.g. corner-[superellipse(1.333)].

For an idea of what each of these do you can use Chrome to look at this Play (or any other supported browser). Otherwise there's a screenshot below: