Add Create mixing recipe to craft IE concrete

kubejs/server_scripts/recipes/create/concrete.js

@@ -0,0 +1,10 @@
+// priority: 0
+
+onEvent("recipes", event => {
+  for (let sand_ingredient of [ "sand", "slag" ]) {
+    event.recipes.createMixing("6x immersiveengineering:concrete",
+      [ `2x #forge:${sand_ingredient}`, "#forge:gravel", "#forge:clay",
+         Fluid.of("minecraft:water", 500) ]);
+  }
+});
+