add piston nix pkg for jelly

runtimes/jelly.nix

@@ -0,0 +1,26 @@
+{pkgs, piston, ...}:
+let
+    pkg = pkgs.jelly;
+in piston.mkRuntime {
+    language = "jelly";
+    version = pkg.version;
+    aliases = [];
+
+    run = ''
+        ${pkg}/bin/jelly fu "$@"
+    '';
+
+    tests = [
+        (piston.mkTest {
+            files = {
+                "test.jelly" = ''
+                    “OK”
+                '';
+            };
+            args = [];
+            stdin = "";
+            packages = [];
+            main = "test.jelly";
+        })
+    ];
+}