feat: add Sequence.each method for side-effect iteration without list creation

Add a new `each` method to the Sequence class that iterates over elements
calling a provided function for each, serving as a side-effect-only
alternative to `map` when the resulting list is not needed. This is
particularly important for the upcoming change where `map` will return a
new sequence instead of mutating in place, as demonstrated by updating
the README and index examples to use `each` with Fiber.yield. Includes
documentation in the Sequence API reference and three new test cases
covering normal iteration, empty sequences, and non-function argument
error handling.

src/vm/wren_core.c

@@ -72,6 +72,12 @@ static const char* libSource =
 "    return result\n"
 "  }\n"
 "\n"
+"  each(f) {\n"
+"    for (element in this) {\n"
+"      f.call(element)\n"
+"    }\n"
+"  }\n"
+"\n"
 "  map(f) {\n"
 "    var result = new List\n"
 "    for (element in this) {\n"