Support iso-year-week-day in calendar_leap_year()

[DavisVaughan]

library(clock)

x <- iso_year_week_day(2020:2030)
x
#> <iso_year_week_day<year>[11]>
#>  [1] "2020" "2021" "2022" "2023" "2024" "2025" "2026" "2027" "2028" "2029"
#> [11] "2030"

calendar_leap_year(x)
#>  [1]  TRUE FALSE FALSE FALSE FALSE FALSE  TRUE FALSE FALSE FALSE FALSE

x[calendar_leap_year(x)]
#> <iso_year_week_day<year>[2]>
#> [1] "2020" "2026"


# Test that it works right.
# Exactly 71 leap weeks for any 400 year cycle.
start <- 1900

while (start < 2000) {
  end <- start + 400 - 1
  x <- seq(start, end)
  x <- iso_year_week_day(x)
  
  if (sum(calendar_leap_year(x)) != 71) {
    stop("something is horribly wrong!")
  }
  
  start <- start + 1
}

^{Created on 2023-04-21 with reprex v2.0.2.9000}