Suppport CURRENT_TIMESTAMP with pricision

[akihiro17]

This PR fixes the following error when parsing the CURRENT_TIMESTAMP with pricision.

CREATE TABLE t1
(
  ts timestamp(6) DEFAULT CURRENT_TIMESTAMP(6) ON UPDATE CURRENT_TIMESTAMP(6),
  dt datetime(6) DEFAULT CURRENT_TIMESTAMP(6) ON UPDATE CURRENT_TIMESTAMP(6)
)

Failure/Error: do_parse

     Racc::ParseError:

       parse error on value "(" (S_LEFT_PAREN)
     # (eval):3:in `_racc_do_parse_c'
     # (eval):3:in `do_parse'
     # ./lib/mysql-parser/lexer.rb:29:in `scan_str'
     # mysql.y.rb:1349:in `parse'
     # ./spec/parser_spec.rb:192:in `block (2 levels) in <class:MySQLParserTester>'

ref. https://dev.mysql.com/doc/refman/8.0/en/timestamp-initialization.html

[akihiro17]

@michaelfinch What do you think about this change?