This commit introduces two new flags for jsonb assignment:

* JB_PATH_FILL_GAPS: Appending array elements on the specified position, gaps
  are filled with nulls (similar to the JavaScript behavior).  This mode also
  instructs to   create the whole path in a jsonb object if some part of the
  path (more than just the last element) is not present.

* JB_PATH_CONSISTENT_POSITION: Assigning keeps array positions consistent by
  preventing prepending of elements.

Both flags are used only in jsonb subscripting assignment.

Initially proposed by Nikita Glukhov based on polymorphic subscripting
patch, but transformed into an independent change.

Discussion: https://postgr.es/m/CA%2Bq6zcV8qvGcDXurwwgUbwACV86Th7G80pnubg42e-p9gsSf%3Dg%40mail.gmail.com
Discussion: https://postgr.es/m/CA%2Bq6zcX3mdxGCgdThzuySwH-ApyHHM-G4oB1R0fn0j2hZqqkLQ%40mail.gmail.com
Discussion: https://postgr.es/m/CA%2Bq6zcVDuGBv%3DM0FqBYX8DPebS3F_0KQ6OVFobGJPM507_SZ_w%40mail.gmail.com
Discussion: https://postgr.es/m/CA%2Bq6zcVovR%2BXY4mfk-7oNk-rF91gH0PebnNfuUjuuDsyHjOcVA%40mail.gmail.com
Author: Dmitry Dolgov
Reviewed-by: Tom Lane, Arthur Zakirov, Pavel Stehule, Dian M Fay
Reviewed-by: Andrew Dunstan, Chapman Flack, Merlin Moncure, Peter Geoghegan
Reviewed-by: Alvaro Herrera, Jim Nasby, Josh Berkus, Victor Wagner
Reviewed-by: Aleksander Alekseev, Robert Haas, Oleg Bartunov