[stdlib] Add an option for thousands separator to package "format"

[trs123]

The primitive Format from stdlib's package "format" should allow for formatting with thousands separators, e.g. "10,000,000" instead of "10000000".

Note
Of course the thousands separator should be language dependent (and I just learned that even the grouping is language/culture dependent) but i18n seems not supported yet by Pony. I assume that currently the package "format" uses "." as decimal separator and therefore could use "," as grouping separator with a grouping of 3 digits for now (thousands separator). To allow more freedom the thousands separator to be used could be provided instead.

Proposal
The basic function Format.apply() could by extended with a new parameter separator (or maybe sep as Pony APIs seem to prefer abbreviated variable names):

"""
...
* sep. The character to be used as thousands separator.
"""
fun apply(
    str: String,
    fmt: FormatDefault = FormatDefault,
    prefix: PrefixDefault = PrefixDefault,
    prec: USize = -1,
    width: USize = 0,
    align: Align = AlignLeft,
    fill: U32 = ' ',
    sep: (U32 | None) = None)
    : String iso^