A cleanup for MDEV-13916 Enforce check constraint on JSON type

1. Renaming Type_handler_json to Type_handler_json_longtext
   There will be other JSON handlers soon, e.g. Type_handler_json_varchar.

2. Making the code more symmetric for data types:

- Adding a new virtual method
  Type_handler::Column_definition_validate_check_constraint()
- Moving JSON-specific code from sql_yacc.yy to
  Type_handler_json_longtext::Column_definition_validate_check_constraint()

3. Adding new files sql_type_json.cc and sql_type_json.h
   and moving Type_handler+JSON related code into these files.

sql/field.h

@@ -4675,6 +4675,7 @@ public:
   bool fix_attributes_bit();
 
   bool check(THD *thd);
+  bool validate_check_constraint(THD *thd);
 
   bool stored_in_db() const { return !vcol_info || vcol_info->stored_in_db; }