Add Array#truncate

[HertzDevil]

Closes #10665.

[jkthorne]

I am curious. Why no negative numbers? could negative numbers indicate direction?

example:

array = [1, 2, 3, 4, 5]

array.truncate(array.size - 1, -3) # => [3, 4, 5]

[HertzDevil]

That convention has never been adopted by any other methods on Indexable:

array = [1, 2, 3, 4, 5]

array[array.size - 1, -3] # Unhandled exception: Negative count: -3 (ArgumentError)

[asterite]

Looks great!

[straight-shoota]

Thanks @HertzDevil