Add tasks for all the new languages

Task/Singly-linked-list-Element-insertion/Sidef/singly-linked-list-element-insertion.sidef

@@ -0,0 +1,18 @@
+func insert_after(a,b) {
+    b{:next} = a{:next};
+    a{:next} = b;
+}
+
+var B = Hash.new(
+    data => 3,
+    next => nil,    # not a circular list
+);
+var A = Hash.new(
+    data => 1,
+    next => B,
+);
+var C = Hash.new(
+    data => 2,
+);
+
+insert_after(A, C);