Add Format()

[mcdee]

This adds a Format() function to decimal, allowing for setting of a thousands separator (as requested in #267) and decimal separator, as well as the existing functionality for trimming trailing zeros. This allows for easier use in localized environments.

This replaces the prior internal string() function, which could now be removed entirely but that would result in a larger PR so has been left for now.

Worth taking a close look at lines 1032-1034 to ensure they behave as expected, as they alter the flow compared to before this PR.

[mwoss]

Thanks for the contribution and I deeply apologize for my late review. 🙏

The ability to format the string representation seems like a nice addition to the library. If I remember correctly, this feature has been already requested in the past.
Sadly, there's one big problem. We cannot use the Format as a method name. Go does not support method overloading and there's the fmt.Formatter interface that needs to be implemented on Decimal struct one day. Implementing this interface would allow the comprehensive use of decimals in strings. So I think we should not block it by using this name.

I'm also thinking about whether localized-like Format method should be a part of the Decimal struct. If we would implement fmt.Formatter, users could freely use packages like golang.org/x/text/message to format the output to their needs. They offer more advanced formatting options and probably would fulfill all current and future devs needs. What do you think?