proposal: x/text: provide an option for compact number formatting

[nkall]

The Unicode Technical Standard specifies a list of long and short Compact Number Formats for use in formatting truncated versions of numbers (e.g. 35K, 2M). For reference, the JSON representation of these formats in English can be viewed here. x/text implements a lot of the standard, including currency and number formatting, but there isn't currently a way to get compact formats as described.

I suggest x/text/number be given a Compact Option which can be created with either a "long" or "short" typed argument. When this option is provided and the number formatted is suitable for compacting in that locale, the compact version should instead be returned.

[rsc]

/cc @mpvl

[mpvl]

This naturally fits within x/text.

The infrastructure to parse and organize the needed data is present in x/text's internal packages.

I would just not make it an option, but rather a Formatter. This way formatting w compact number would read:

p := message.NewPrinter(language.English)

p.Printf("%v bottles of beer on the wall.", number.Compact(3000000))
// Prints: 3M bottles of beer on the wall.

This would default to english printing when used with the fmt package.