Replace upvalue-based shell dispatch with OP_LUSH opcode

Add a dedicated OP_LUSH A B instruction that loads shell C functions
directly from the registry, eliminating the need for upvalue allocation
in mainfunc(), upvalue population in lua_load(), and the fragile
LUSH_NUM_UPVALS heuristic. Every chunk no longer carries 4 extra
upvalues.

lopnames.h

@@ -94,6 +94,7 @@ static const char *const opnames[] = {
   "SETLIST",
   "CLOSURE",
   "VARARG",
+  "LUSH",
   "GETVARG",
   "ERRNNIL",
   "VARARGPREP",