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.

src/wren_core.c

@@ -64,6 +64,16 @@ static const char* libSource =
 "    }\n"
 "    return true\n"
 "  }\n"
+"\n"
+"  reduce(acc, f) {\n"
+"    for (element in this) {\n"
+"      acc = f.call(acc, element)\n"
+"    }\n"
+"    return acc\n"
+"  }\n"
+"\n"
+"  reduce(f) { this[1..-1].reduce(this[0], f) }\n"
+"\n"
 "}\n"
 "\n"
 "class List is Sequence {\n"