code: func IsKeyword(literal string) (Token, bool) {
  	if keyword, exists := keywordTable[literal]; exists {
  		if keyword.futureKeyword {
  			return KEYWORD, keyword.strict
  		}
  		return keyword.token, false
  	}
  	return 0, false
  }
docs: // IsKeyword returns the keyword token if literal is a keyword, a KEYWORD token
  // if the literal is a future keyword (const, let, class, super, ...), or 0 if the literal is not a keyword.
  //
  // If the literal is a keyword, IsKeyword returns a second value indicating if the literal
  // is considered a future keyword in strict-mode only.
  //
  // 7.6.1.2 Future Reserved Words:
  //
  //       const
  //       class
  //       enum
  //       export
  //       extends
  //       import
  //       super
  //
  // 7.6.1.2 Future Reserved Words (strict):
  //
  //       implements
  //       interface
  //       let
  //       package
  //       private
  //       protected
  //       public
  //       static
  //