Add tab completion for the WAIT FOR LSN MODE option

Add tab completion for the WAIT FOR LSN MODE option

Update psql tab completion to support the optional MODE option in the
WAIT FOR LSN command.  After specifying an LSN value, completion now offers
both MODE and WITH keywords.  The MODE option specifies which LSN type to wait
for.  In particular, it controls whether the wait is evaluated from the
standby or primary perspective.

When MODE is specified, the completion suggests the valid mode values:
standby_replay, standby_write, standby_flush, and primary_flush.

Discussion: https://postgr.es/m/CABPTF7UiArgW-sXj9CNwRzUhYOQrevLzkYcgBydmX5oDes1sjg%40mail.gmail.com
Author: Xuneng Zhou <xunengzhou@gmail.com>
Reviewed-by: Alexander Korotkov <aekorotkov@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Alvaro Herrera <alvherre@kurilemu.de>