Add test cases demonstrating that the reduce function works with string arrays
and concatenation, verifying both with and without initial accumulator values.
The test uses a list of characters ["W", "o", "r", "l", "d"] and the existing
sum function to show that reduce correctly folds strings in order.
Implement `reduce` on the `Sequence` class in core.wren with two overloads: one taking an initial accumulator and a function, and another using the first element as the initial accumulator. Add three test files covering normal reduction, single-element edge case, and wrong arity error.
