Add the `fmt_partsper()` function

[janekbennett]

There is a fmt_percent formatting function in gt but, in the same spirit, it would be lovely to have a an additional general function that does parts-per notation (eg. ppm, ppb, ppt, ppmV, per mille and per ten thousand, to name a few). These types of units are pretty common in the sciences and so this function would definitely get a lot of use from the scientific community.

[rich-iannone]

Thanks for posting the issue, it's a great feature request. I used to use this type of notation and I'm sure many would appreciate support for this.

I also like the function name you proposed. I think ppm, ppb, etc. should be part of fmt_partsper(). For per mille and per ten thousand, that might separate go into a fmt_per() function. For the first, we can go from ppm to ppmV by way of the pattern argument.