Allow readonly array in where

[ftzi]

Allows

const array = ['a', 'b'] as const;
query(doc, [where('key', 'in', array)])

Without this PR, an error is throw:

No overload matches this call.
  The last overload gave the following error.
    Argument of type 'string' is not assignable to parameter of type '[keyof DbOrder, string | number | symbol, string | number | symbol, string | number | symbol, string | number | symbol, string | number | symbol, string | number | symbol, string | ... 1 more ... | symbol, string | ... 1 more ... | symbol, string | ... 1 more ... | symbol]'.