Add support for self referencing selectors

[joseDaKing]

Add support for self-referencing selectors.

const spaceX = style({
    selectors: {
        "& > * ~ *": {
            marginLeft: `calc(${value} * calc(1 - ${fallbackVar(spaceXReverseVar, "0")}))`,
            marginRight: `calc(${value} * ${fallbackVar(spaceXReverseVar, "0")})`,
        }
    } 
})

The selector will reference spaceX className.