Suppport CURRENT_TIMESTAMP with pricision

[akihiro17]

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

CREATETABLEt1
(
ts timestamp(6) DEFAULT CURRENT_TIMESTAMP(6) ONUPDATECURRENT_TIMESTAMP(6),
dt datetime(6) DEFAULT CURRENT_TIMESTAMP(6) ONUPDATECURRENT_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?