feat: add length function.

Takes a list and an integer, splits off n elements from the list
and returns a list where the first element contains the first n
elements of the original list, the cdr of that list contains the
rest.

Jellyfin.Plugin.SmartPlaylist/Lisp/Interpreter.cs

@@ -62,6 +62,25 @@ namespace Jellyfin.Plugin.SmartPlaylist.Lisp {
                     (rev-helper lst '())))
                 """
             );
+            this["split"] = e.eval(
+                """
+                (lambda
+                  (lst n)
+                  (if
+                    (or (= n 0) (null lst))
+                    (cons '() lst)
+                    (let
+                      (s (split (cdr lst) (- n 1)))
+                      (cons (cons (car lst) (car s)) (cdr s)))))
+                """
+            );
+            this["length"] = e.eval(
+                """
+                (lambda
+                  (lst n)
+                  (if (null lst) n (length (cdr lst) (+ n 1))))
+                """
+            );
             this["qsort"] = e.eval(
                 """
                 (lambda