feat: add debug print for CALL_EXTERN opcode

src/vm/fzl_vm_debug.c

@@ -1,6 +1,7 @@
 #include <common/fzl_opcodes.h>
 #include <common/fzl_value.h>
 #include <stdbool.h>
+#include <stddef.h>
 #include <stdint.h>
 #include <stdio.h>
 #include <vm/fzl_vm.h>
@@ -22,6 +23,14 @@ void fzl_vm_print_ops(fzl_vm_t *vm)
 
     switch (vm->reader->ops[i])
     {
+      case FZL_OPCODE_CALL_EXTERN:
+      {
+        size_t fn_n = vm->reader->ops[++i];
+        printf("CALL_EXTERN FN=#%zu (%s)\n", fn_n,
+               vm->reader->extern_fns[fn_n].used_name);
+        break;
+      }
+
       case FZL_OPCODE_STORE_LOCAL:
       {
         size_t depth = vm->reader->ops[++i];