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.

builtin/core.wren

@@ -26,6 +26,12 @@ class Sequence {
     return result
   }
 
+  each(f) {
+    for (element in this) {
+      f.call(element)
+    }
+  }
+
   map(f) {
     var result = new List
     for (element in this) {