Add assert_count_true() to verify that an expected number of values are TRUE

[billdenney]

Related to #539

I use a variant of this often in my data pipelines where I want to select a certain number of values and be sure that I'm only getting the number I expect. For example, I want to change one date for one person, and I want to be sure that I only change that one date:

# We are very young
my_data <- data.frame(name = c("Bill", "Sam"), birthdate = c("2024-05-22", "2024-05-22"))
my_data |>
  dplyr::mutate(
    birthdate =
      dplyr::case_when(
        assert_count_true(name == "Bill" & birthdate == "2024-05-22") ~ "2024-05-23",
        TRUE ~ birthdate
      )
  )