Z-Index Tokens

Z-Index tokens to use within swirl.

Token Name	Value (Z-Index)
	`z-0`	`0`
	`z-10`	`10`
	`z-20`	`20`
	`z-30`	`30`
	`z-40`	`40`
	`z-50`	`50`
	`z-auto`	`auto`