feat: implement List.addAll() method to append elements from iterable

Add `addAll(other)` method to the List class in both the Wren core library source and its embedded C string representation. The method iterates over the given `other` sequence, appends each element to the list using the existing `add()` method, and returns the original `other` argument to match the documented return behavior. Include a new test file `test/list/add_all.wren` covering basic appending, empty iterable, range input, and return value verification.

builtin/core.wren

@@ -17,6 +17,13 @@ class Sequence {
 }
 
 class List is Sequence {
+  addAll(other) {
+    for (element in other) {
+      add(element)
+    }
+    return other
+  }
+
   toString {
     var result = "["
     for (i in 0...count) {