feature: enable users to pass arrays to Decimal.sum

[hunterwilhelm]

Fixes #230

A minor conflict with #254 will happen. By choosing "Accept Combination", it will easily be resolved, since the changed lines don't overlap, they are just close enough to trigger it.

[MikeMcl]

Hello.

I'll consider this change, but I am uncomfortable with its inconsistency with other functions such as Decimal.max.

There is also the question of whether, for example, Decimal.sum(1, [2, 3]) should be supported to, as it doesn't really "enable users to pass arrays to Decimal.sum", it only enables them to pass one array as the first argument.

Also, this library is advertised as using ECMAScript 3 features only, and Array.isArray would break that.

Thank you for doing a careful job with the docs, test, and types updates.