Add tasks for all the new languages

Task/Singly-linked-list-Element-insertion/EchoLisp/singly-linked-list-element-insertion.echolisp

@@ -0,0 +1,11 @@
+(define (insert-after lst target item)
+    (when (null? lst) (error "cannot insert in" null))
+    (let [(sub-list (member target lst))]
+        (if sub-list (set-cdr! sub-list (cons item (cdr sub-list))) ; make chirurgy if found
+        (nconc lst item)))) ; else append item
+
+(define L '(a b))
+(insert-after L 'a 'c)
+    L  → (a c b)
+(insert-after L 'x 'y)
+    L → (a c b y)