Add ARD function to calculate cumulative counts

[edelarua]

Create function to calculate statistics returned by tern::s_count_cumulative().

Given threshold(s), a direction (greater or less than), and whether to include the threshold value(s), return counts and fractions of values of a numeric variable in each given threshold.

See https://insightsengineering.github.io/tern/latest-tag/reference/count_cumulative.html for details on the tern function.

[ddsjoberg]

I think for now, we can split what that tern function did into two parts: the first does the preprocessing of creating a new variable with the thresholds supplied, and the second that returns the tabulations with the cumulative counts/percentages.

The first part we can skip for now, and for the second we'll update cards::ard_categorical() to optionally return cum_n and cum_p. This is being tracked in this issue FYI insightsengineering/cards#145