Add the fmt_fraction() formatter function

[rich-iannone]

This adds the fmt_fraction() formatter, which allows for flexible formatting of numerical values to mixed fractions of configurable accuracy. Example:

library(gt)
library(tidyverse)

# Define values
range_0_1 <- c(0.0001, 0.001, 0.01, 0.1, 0.25, 0.4, 0.5, 0.6, 0.75, 0.9, 0.99, 0.999, 0.9999)
range_0_1_minus <- range_0_1 * -1
range_1_2 <- range_0_1 + 1
range_1_2_minus <- range_0_1_minus - 1
exact_numbers <- c(-1, 0, 1)
not_numbers <- c(NA_real_, NaN, Inf, -Inf)

# Generate a gt table that demonstrate use of `fmt_fraction()`
dplyr::tibble(
  x = c(
    range_0_1, range_0_1_minus, range_1_2, range_1_2_minus,
    exact_numbers, not_numbers
  ),
  category = c(
    rep("0 to 1", length(range_0_1)),
    rep("0 to -1", length(range_0_1_minus)),
    rep("1 to 2", length(range_1_2)),
    rep("-1 to -2", length(range_1_2_minus)),
    rep("Exact Numbers", length(exact_numbers)),
    rep("Not Numbers", length(not_numbers))
  ),
  low = x,
  med = x,
  high = x,
  halves = x,
  quarters = x,
  eighths = x,
  sixteenths = x,
  hundredths = x
) %>%
  gt(rowname_col = "x", groupname_col = "category") %>%
  fmt_fraction(columns = low, accuracy = "low") %>%
  fmt_fraction(columns = med, accuracy = "med") %>%
  fmt_fraction(columns = high, accuracy = "high") %>%
  fmt_fraction(columns = halves, accuracy = "/2") %>%
  fmt_fraction(columns = quarters, accuracy = "/4") %>%
  fmt_fraction(columns = eighths, accuracy = "/8") %>%
  fmt_fraction(columns = sixteenths, accuracy = "/16") %>%
  fmt_fraction(columns = hundredths, accuracy = "/100") %>%
  cols_width(everything() ~ px(100)) %>%
  opt_all_caps()

Fixes: #402