Add the fmt_partsper() function

[rich-iannone]

With the new fmt_partsper() function, we can now format values as high-ratio, dimensionless quantities. These are less common that percentage values but no less important (some of these are used extensively, depending on the field of study). The list of formats (with their associated keywords) are:

• "per-mille": Per mille, (1 part in 1,000)
• "per-myriad": Per myriad, (1 part in 10,000)
• "pcm": Per cent mille (1 part in 100,000)
• "ppm": Parts per million, (1 part in 1,000,000)
• "ppb": Parts per billion, (1 part in 1,000,000,000)
• "ppt": Parts per trillion, (1 part in 1,000,000,000,000)
• "ppq": Parts per quadrillion, (1 part in 1,000,000,000,000,000)

The function is structured similarly to fmt_percent() but there is some flexibility here to provide a replacement symbol if needed (e.g., ppm -> ppmV).

Here is an example with per mille formatting:

library(gt)
library(tidyverse)

dplyr::tibble(x = 0:-5, a = 10^(0:-5), b = a) %>%
  gt(rowname_col = "x") %>%
  fmt_scientific(a, decimals = 0) %>%
  fmt_partsper(
    columns = b,
    to_units = "per-mille"
  )

Fixes: #608

[jcheng5]

1. Rename fmt_ratio to fmt_partsper
2. Take out the changes to context_symbol_str, leave that to (eventually) be currency-only