Export `stop_unsupported()`

[DavisVaughan]

For extension packages that implement some of vec_math(), but not all of it

[DavisVaughan]

Although this seems to work fairly well too (at least for rcrd types which error by default on all math functions)

#' @export
vec_math.clock_calendar <- function(.fn, .x, ...) {
  switch(
    .fn,
    is.nan = calendar_is_nan(.x),
    is.finite = calendar_is_finite(.x),
    is.infinite = calendar_is_infinite(.x),
    NextMethod()
  )
}