Add support for UNIX socket connection to PostgreSQL

[TobiX]

Minimal prototype. I tried to unify the connection code into one place
(src/helpers/db.ts), since Drizzle seems to need slightly different config in
different places (see the issue linked from #343).

Closes #343

Slightly tested in dev mode, migrations work, application starts.

Caveat: This uses the host parameter instead of the path parameter (see
https://github.com/porsager/postgres#connection-details), since the connection
code (https://github.com/porsager/postgres/blob/master/src/index.js#L468)
builds the "real" socket name from that. If you want, I can extend the code to
check for the path to make this a little bit more "auto-magic".

Feel free to close if you don't want this feature.

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[TobiX]

I have read the CLA Document and I hereby sign the CLA