Add support for currency conversion for numerical steno input containing commas

[stanographer]

Summary of changes

GOAL

Currency formatting currently only accepts pure digit inputs (3400000 for $3,400,000). Some stenographers write numbers using constructive shortcuts that glue digits with commas included.

THO*U = {^},000
F*UNT = {^},500,000
M*L = {^},000,000

We want to be able to properly format currency amounts when the input is 23,000 the same as 23000.

This doesn't work with the currency-formatting macro currently because commas break casting.(https://github.com/openstenoproject/plover/blob/master/plover/meta/currency.py#L11)

APPROACH

1. Check if the contents of last_words is numerical i.e. is a string that contains at least one number.
2. If it does, search for, and replace every instance of , and pass it along as the cast_input.
3. Else, just pass the function the original string as it would originally.

Closes

This wasn't an official issue to my knowledge.

Pull Request Checklist

• Changes have tests
  (https://github.com/openstenoproject/plover/pull/1391/files#diff-09db2cc1c6dd18f0e430beac98d6e7dd640c7493bf0d412625ff32687cf02a61R758)
• News fragment added in news.d. See documentation for details
  https://github.com/openstenoproject/plover/pull/1391/files#diff-51920e95310ebfbc1ae31709f3b95f89afffbf4f1a6e38e8b2b406e2fb6197eaR46

[morinted]

Thank you @stanographer !