Add `nth-*` variants

[thecrypticace]

This adds variants for the :nth-child(), :nth-last-child(), :nth-of-type(), and :nth-last-of-type() pseudo classes. These variants support arbitrary values as well as numeric bare values. Refer to the table below for example usasges on what selectors these variants produce:

Variant	Selector
nth-3:*	&:nth-child(3)
nth-[2n+1]:*	&:nth-child(2n+1)
nth-[2n+1_of_.item]:*	&:nth-child(2n+1 of .item)
nth-last-3:*	&:nth-last-child(3)
nth-last-[2n+1]:*	&:nth-last-child(2n+1)
nth-last-[2n+1_of_.item]:*	&:nth-last-child(2n+1 of .item)
nth-of-type-3:*	&:nth-of-type(3)
nth-of-type-[2n+1]:*	&:nth-of-type(2n+1)
nth-last-of-type-3:*	&:nth-last-of-type(3)
nth-last-of-type-[2n+1]:*	&:nth-last-of-type(2n+1)