Add ellipsis to `tally()` and `add_tally()`

[krlmlr]

The wt, sort and name arguments should be behind an ellipsis, like for count() and similarly for other verbs.

Agent instructions:

Add an ellipsis to the generic and to the method. Ensure that users still can pass unnamed arguments, with a compatibility warning. For this, implement a pattern like

tally <- function(x, ..., wt = NULL ......) {
  # Add compat code that checks ... and converts to regular arguments
  tally_(x, wt = !!wt, sort = sort, name = name)
}
tally_ <- function(.....) {
  UseMethod("tally")
}

Add comprehensive tests for all combinations of named and unnamed args, perhaps with a tester class that has a tally() method that just returns the args.