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.wren

@@ -0,0 +1,9 @@
+var a = [1, 4, 2, 1, 5]
+var max = new Fn {|a, b| a > b ? a : b }
+var sum = new Fn {|a, b| a + b }
+
+IO.print(a.reduce(max)) // expect: 5
+IO.print(a.reduce(10, max)) // expect: 10
+
+IO.print(a.reduce(sum)) // expect: 13
+IO.print(a.reduce(-1, sum)) // expect: 12