Make saner the tab completion of INSERT and DELETE in psql

Make saner the tab completion of INSERT and DELETE in psql

When specified directly as DML queries, INSERT was not getting always
completed to "INSERT INTO", same for DELETE with "DELETE FROM".  This
makes the completion behavior more consistent for both commands, saving
a few keystrokes.

Commands on policies, triggers, grant/revoke, etc. require only DELETE
as completion keyword.

Author: Haiying Tang
Reviewed-by: Dilip Kumar, Julien Rouhaud
Discussion: https://postgr.es/m/OS0PR01MB61135AE2B07CCD1AB8C6A0F6FB549@OS0PR01MB6113.jpnprd01.prod.outlook.com