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.

builtin/core.wren

@@ -21,6 +21,16 @@ class Sequence {
     }
     return true
   }
+
+  reduce(acc, f) {
+    for (element in this) {
+      acc = f.call(acc, element)
+    }
+    return acc
+  }
+
+  reduce(f) { this[1..-1].reduce(this[0], f) }
+
 }
 
 class List is Sequence {