I realise that this is supposed to be a port of rust's format!, which does not support grouping digits (as in 1,000,000.0000), but I maybe this feature is commonly wanted in typesetting and can be justified.

(Example use case: At the moment, I'm having to load and combine both oxifmt and metro to format numbers with 1) thousands separators 2) an exact number of decimal digits. If this was added to oxifmt, then it alone would be sufficient!)