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.

test/list/add.wren

@@ -1,6 +1,4 @@
-var a = []
-a.add(1)
-IO.print(a) // expect: [1]
+var a = [1]
 a.add(2)
 IO.print(a) // expect: [1, 2]
 a.add(3)

test/list/add_all.wren

@@ -0,0 +1,11 @@
+var a = [1]
+a.addAll([2, 3])
+IO.print(a) // expect: [1, 2, 3]
+a.addAll([])
+IO.print(a) // expect: [1, 2, 3]
+a.addAll(4..6)
+IO.print(a) // expect: [1, 2, 3, 4, 5, 6]
+
+// Returns argument.
+var range = 7..9
+IO.print(a.addAll(range) == range) // expect: true