Add tasks for all the new languages

Task/Stack/Sidef/stack-2.sidef

@@ -0,0 +1,10 @@
+class Stack(stack=[]) {
+    method pop        { stack.pop };
+    method push(item) { stack.push(item) };
+    method empty      { stack.is_empty };
+}
+
+var stack = Stack();
+stack.push(42);
+say stack.pop;          # => 42
+say stack.empty;        # => true