Implement standard datetime parsing mode

SQL Standard 2016 defines rules for handling separators in datetime template
strings, which are different to to_date()/to_timestamp() rules.  Standard
allows only small set of separators and requires strict matching for them.

Standard applies to jsonpath .datetime() method and CAST (... FORMAT ...) SQL
clause.  We're not going to change handling of separators in existing
to_date()/to_timestamp() functions, because their current behavior is familiar
for users.  Standard behavior now available by special flag, which will be used
in upcoming .datetime() jsonpath method.

Discussion: https://postgr.es/m/CAPpHfdsZgYEra_PeCLGNoXOWYx6iU-S3wF8aX0ObQUcZU%2B4XTw%40mail.gmail.com
Author: Alexander Korotkov