code: function getKyouikuGrade(ch) {
      let grade;
  
      if (includes(kyouikuKanji.grade1, ch)) {
          grade = 1;
      }
      else if (includes(kyouikuKanji.grade2, ch)) {
          grade = 2;
      }
      else if (includes(kyouikuKanji.grade3, ch)) {
          grade = 3;
      }
      else if (includes(kyouikuKanji.grade4, ch)) {
          grade = 4;
      }
      else if (includes(kyouikuKanji.grade5, ch)) {
          grade = 5;
      }
      else if (includes(kyouikuKanji.grade6, ch)) {
          grade = 6;
      }
  
      return grade;
  }
docs: return the grade of a kyouiku kanji