jei_hides: use .concat instead of array-spread (Rhino compat). 0.21.0 -> 0.21.1.

KubeJS uses Rhino as its JS engine, which doesn't support array-spread
syntax (`[...a, ...b]`). The line `[...WAYSTONE_ITEMS, ...COIN_ITEMS]
.forEach(...)` threw EvaluatorException: syntax error at jei_hides.js:46
on client launch, surfacing the KubeJS client script errors dialog.

Fix: use Array.prototype.concat() which is universal JS. Also extracted
the hide-quietly helper into a named function for readability.

pack/pack.lock.json

@@ -1,7 +1,7 @@
 {
   "$schema": "Brass-and-Sigil pack.lock.json - generated, do not edit by hand unless you know what you are doing",
   "name": "Brass and Sigil",
-  "version": "0.21.0",
+  "version": "0.21.1",
   "minecraft": "1.21.1",
   "loader": {
     "type": "neoforge",