.humanize() should have a numeric option for 1 instead of a/an

### Feature Request

Option to change "a/an" to 1:
* "2 hours and a minute" --> "2 hours and 1 minute"
* "an hour and 12 minutes" --> "1 hour and 12 minutes"
* etc.

example `Arrow.humanize(..., numeric=True)`

Start by locating the Arrow.humanize implementation and its handling of "a/an" for singular units. Review the relevant humanize behavior and existing coverage before deciding how the numeric option should affect the examples; done means numeric=True produces "1 minute" and "1 hour" while the default wording remains unchanged.

Written by the indexing model from the issue text.