PostgreSQL allows `\\\"` as an escape for `\"`. However, we seem to not
allow it at all. This was a blocker for osm2pgsql.

In general, I think we should be allowing any non-captured escape to be
used as is, as PostgreSQL does:
```
otan=# \copy copytest from stdin
Enter data to be copied followed by a newline.
End with a backslash and a period on a line by itself, or an EOF signal.
>> \1\2
>> COPY 1
otan=# select * from copytest
;
    t
----------
 \"
 \x01\x02
(2 rows)

otan=# \copy copytest from stdin
Enter data to be copied followed by a newline.
End with a backslash and a period on a line by itself, or an EOF signal.
>> \a
>> COPY 1
otan=# select * from copytest
;
    t
----------
 \"
 \x01\x02
 a
(3 rows)

otan=# \copy copytest from stdin
Enter data to be copied followed by a newline.
End with a backslash and a period on a line by itself, or an EOF signal.
>> \b
>> COPY 1
otan=# select * from copytest
;
    t
----------
 \"
 \x01\x02
 a
 \x08
(4 rows)
```

If you think we should change our code to follow this, let me know.

Release note: None