fix(compiler): prevent schema parse failures by ignoring psql meta commands

[andrewmbenton]

This is kind of a lazy quick-and-dirty implementation that doesn't attempt to consolidate some duplicate line parsing that we do during migration removal and just sticks a func right in the package that handles file parsing for all engines, even though this line filter only applies to postgresql.

Happy to revise if there's a better place to put this.

Resolves https://github.com/sqlc-dev/sqlc/issues/4065

[atombender]

Can this please be merged? It's a blocking issue with Postgres 17. @kyleconroy

[j-houston]

Bump. Can we get this merged?

[ignat980]

I added https://github.com/sqlc-dev/sqlc/pull/4177 to address @gbarr comment and not to break any other existing postgresql workflows (i.e., on /connect or other psql meta commands)