support psql commands

[mmoncure]

psql commands \if, \set etc are normal to see in .sql source files in order to support certain kinds of build managements systems. Currently they are thrown out as unrecognized. I guess this is a bit of a stretch to be called a bug, but in general the checker should parse .sql that psql can parse. backslash commands are very simple; if the first character of a line is a backslash the rest of the line can be ignored. I not believe there is any need to interpret the commands.

To Reproduce

This file:
psql_test.sql

\if 1

do $$
begin
  raise notice '%', $q$
\this is in a string;
  $q$;

end;
$$;

    \endif

...yields...

./psql_test.sql:1:1 syntax ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ Invalid statement: syntax error at or near "\"

  > 1 │ \if 1
      │ ^^^^^
    2 │
    3 │ do $$


./psql_test.sql:12:5 syntax ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ Invalid statement: syntax error at or near "\"

    10 │ $$;·
    11 │
  > 12 │     \endif
       │     ^^^^^^
    13 │


Checked 1 file in 3ms. No fixes applied.
Found 2 errors.
check ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━


  ✖ Some errors were emitted while running checks.

psql however yields:

NOTICE:
\this is in a string;

DO

[psteinroe]

Good point, will add that!