Add the ability to sort classes in the result list

[svyatov]

Currently, the result order depends on the argument structure. It should be trivial to add a sort option, which might be helpful, for example, in tests.

The easiest way is to introduce a new helper because it's hard to pass an argument to the current helper, given that we treat all the arguments as input for the helper.

So...

clsx -> no change
clsxs -> new helper (s suffix stands for 'sorted')
cn -> no change
cns -> new alias for clsxs (again, s stands for 'sorted')