Add tasks for all the new languages

Task/Longest-common-subsequence/Sidef/longest-common-subsequence.sidef

@@ -0,0 +1,16 @@
+func lcs(xstr, ystr) is cached {
+
+    xstr.is_empty && return xstr;
+    ystr.is_empty && return ystr;
+
+    var(x, xs, y, ys) = (xstr.ft(0,0), xstr.ft(1),
+                         ystr.ft(0,0), ystr.ft(1));
+
+    if (x == y) {
+        x + lcs(xs, ys)
+    } else {
+        [lcs(xstr, ys), lcs(xs, ystr)].max_by { .len };
+    }
+}
+
+say lcs("thisisatest", "testing123testing");