pgsql: Accept fractional seconds in jsonpath's datetime() method.

Accept fractional seconds in jsonpath's datetime() method.

Commit 927d9abb6 purported to make datetime() accept any string
that could be output for a datetime value by to_jsonb(). But it
overlooked the possibility of fractional seconds being present,
so that cases as simple as to_jsonb(now()) would defeat it.

Fix by adding formats that include ".US" to the list in
executeDateTimeMethod(). (Note that while this is nominally
microseconds, it'll do the right thing for fractions with
fewer than six digits.)

In passing, re-order the list to restore the datatype ordering
specified in its comment. The violation accidentally did not
break anything; but the next edit might be less lucky, so add
more comments.

Per report from Tim Field. Back-patch to v13 where datetime()
was added, like the previous patch.

Discussion: https://postgr.es/m/014A028B-5CE6-4FDF-AC24-426CA6FC9CEE@mohiohio.com

Branch
------
REL_15_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/bd590d1fea1ba9245c791d589eea94d2dbad5a2b

Modified Files
--------------
src/backend/utils/adt/jsonpath_exec.c | 17 +++++++++++++----
src/test/regress/expected/jsonb_jsonpath.out | 15 +++++++++++++++
src/test/regress/sql/jsonb_jsonpath.sql | 3 +++
3 files changed, 31 insertions(+), 4 deletions(-)