fix(types): use correct default export

[plushdohn]

This PR fixes #862 by ensuring classes defined under the postgres namespace aren't also available as a named export, but only under the default postgres. export.

I've tested the change by patching the module in my projects, but I'm not sure if under some specific Typescript scenarios the behaviour could be incorrect.

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Trying to import PostgresError as a named export:

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And here's the correct usage using the default export:

[blechatellier]

Would like this one merged please 👍

[porsager]

Needs some more thumbs up from other typescript users to confirm this doesn't break anything? (i don't use typescript so can't tell)