Add size utilities (shorthand for width and height)

[benface]

It's been suggested a few times, and I think it deserves to be in the core. With this PR, w-4 h-4 could be rewritten as size-4. It follows the same principle as other shorthand utilities such as inset-*, m-*, p-*, etc. Like inset-*, it doesn't use an actual CSS shorthand property since it doesn't exist yet, but there are plans for it. Instead, the new size core plugin sets the width and height properties to the same value, and size-* utilities are overridden by w-* and h-* since those appear later in the CSS.

[adamwathan]

I definitely think this would be convenient but not sure about adding something to core while the official shorthand property name is still in such flux. It's also only one character shorter for most cases:

h-4 w-4
size-4

Going to close for now but can always explore again down the road.