builtins: add parse and to_char_with_style variants for date types

[jseldess]

PR: cockroachdb/cockroach#68351

From release notes:

Introduced to_char(date), which assumes a DateStyle of ISO,MDY and outputs date in that format. There is also a to_char_with_style(date, DateStyle) variant which outputs the date in the chosen DateStyle. This provides an immutable way of casting dates to strings. [#68351][#68351]

[ericharmeling]

We probably don't need to document this built-in explicitly (it's included from the generated docs).