List.add() method.

test/list_add.wren

@@ -0,0 +1,10 @@
+var a = []
+a.add(1)
+io.write(a) // expect: [1]
+a.add(2)
+io.write(a) // expect: [1, 2]
+a.add(3)
+io.write(a) // expect: [1, 2, 3]
+
+// Returns added element.
+io.write(a.add(4)) // expect: 4