Support the \echo command

[mozeryansky]

Description

psql has a \echo command which appears as a syntax error in pgcli.

$ pgcli $DATABASE_URL
Server: PostgreSQL 14.1
Version: 3.4.1
Home: http://pgcli.com
postgres@db:postgres> \echo hi
syntax error at or near "\"
LINE 1: \echo hi

vs

$ psql -d $DATABASE_URL
psql (14.2, server 14.1)
SSL connection (protocol: TLSv1.3, cipher: TLS_AES_256_GCM_SHA384, bits: 256, compression: off)
Type "help" for help.

postgres=> \echo hi
hi