Use duktape to run KaTeX from ruby with Node

Signed-off-by: Danila Fedorin <danila.fedorin@gmail.com>

gemset.nix

@@ -1,4 +1,24 @@
 {
+  duktape = {
+    groups = ["default"];
+    platforms = [];
+    source = {
+      remotes = ["https://rubygems.org"];
+      sha256 = "1nrhvxbhflf2lrd8v4rphsf0lwg0lvfws2i0cpq8s0xxgh4lviia";
+      type = "gem";
+    };
+    version = "2.7.0.0";
+  };
+  execjs = {
+    groups = ["default"];
+    platforms = [];
+    source = {
+      remotes = ["https://rubygems.org"];
+      sha256 = "1yywajqlpjhrj1m43s3lfg3i4lkb6pxwccmwps7qw37ndmphdzg8";
+      type = "gem";
+    };
+    version = "2.9.1";
+  };
   mini_portile2 = {
     groups = ["default"];
     platforms = [];