feat: add Sequence.each method to iterate over elements with a function

Add a new `each` method to the Sequence class in the Wren programming language,
allowing iteration over sequence elements by passing each element to a provided
function. This replaces the previous pattern of using `map` with `Fiber.yield`
in examples, providing a more direct and idiomatic way to perform side effects
per element. The implementation includes a core library method in `core.wren`,
a C source string in `wren_core.c`, documentation in the Sequence markdown file,
and three test cases covering normal iteration, empty sequences, and non-function
argument error handling.

builtin/core.wren

@@ -40,6 +40,12 @@ class Sequence {
     return result
   }
 
+  each(f) {
+    for (element in this) {
+      f.call(element)
+    }
+  }
+
   map(f) {
     var result = new List
     for (element in this) {