feat: add reduce method to Sequence with two overloads and tests

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.

test/list/reduce_single_item.wren

@@ -0,0 +1 @@
+[1].reduce {|a, b| a } // expect runtime error: Range start out of bounds.