Allow for negative formatting of cells with paren

[timkpaine]

Prework

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

Proposal

I would like an argument for:

• GT.fmt_number
• GT.fmt_integer
• GT.fmt_percent
• GT.fmt_scientific
• GT.fmt_currency

to format negative values as (val) instead of -val.

(I am going to try to implement this feature myself and will open a draft PR for help)