Variadic GCD and LCM

While #10 exists, I will use this as a separate tracker for these two algorithms and their variadic structure. Also inspired by Advent of Code 2023 Day 8 pt 2 which used the LCM of a collection of numbers.

The existing `gcd` can be made easily to take a variadic amount of values, along with some checks. `lcm` can also be implemented using the `gcd` method, also variadic.

~~It does allow 0 and 1 parameters, which are easy cases to account for. Just seems funky that one can put `gcd()` and `lcm()` 🤷, but these have sensible mathematical conventions.~~

Start by locating the existing gcd implementation and any related tests in the Swift Numerics repository. Determine the intended variadic signatures and behavior for zero, one, and multiple values from the issue's mathematical conventions. Done means variadic GCD and LCM are implemented through the existing GCD logic and their edge cases are covered by tests.

Written by the indexing model from the issue text.