Add `compact` to `fmt_currency`

[gartinian]

Prework

• [x] Read and abide by the great_tables code of conduct and contributing guidelines.
• [x] Search for duplicates among the existing issues (both open and closed).

Proposal

Add the compact parameter to fmt_currency, so that values such as $12,345 can render as $12K. The intention is to mirror the behavior of compact in other formatters, such as fmt_integer.

I have reviewed the code and believe I know where changes should be made. I am happy to submit a PR if there is alignment on moving forward.