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.
Matt