* Added new "json" type column into Blueprint.php and corresponding grammars:
  - Column definition for type "json" in PostgresGrammar.php as "json".
  - Column definition for type "json" in SqlServerGrammar.php as "nvarchar(max)".
  - Column definition for "json" type in MySqlGrammar.php as "text".
  - Column definition for "json" type in SQLiteGrammar.php as "text".