Consider unused parameters with mix of predicate & action

[hildjj]

With this grammar:

{{
const ONE = Symbol("one");
}}
{
const NUM_T = options.numT ?? 2;
}
foo
  = o:$"o"+ t:$"t"+ &{ return t.length > NUM_T; } !{ return o.length > NUM_T; } {
    return ONE;
  }

o and t show up as unused, because they're not used in the action.