support postgres `json_object` syntax

[lovasoa]

Postgresql supports the following syntax for the json_object function:

json_object('label' VALUE 'value')
json_object('label': 'value')

Reference: https://www.postgresql.org/docs/current/functions-json.html

Related: the key: value syntax was already implemented for mssql by @gaoqiangz in #1507 . We probably just need to enable it (+ a new VALUE separator) for postgres too

#1506

Initially reported in sqlpage/SQLPage#708