Add formatting method (enhancement)

[bdteo]

The default string representation that the Decimal class gives is like this:

(string)Litipk\BigNumbers\Decimal::fromString('11111111111111111.55555555555555')
"11111111111111111.55555555555555"

It would be nice if there is a method to format strings like these to 11,111,111,111,111,111.55555555555555