Use pgtype package instead of subpackage from pgx v3

[vgarvardt]

Hi.

First of all thank you for this package - I find it very useful and easy to use.

Recently I started migration of my project to the latest pgx/v4 and this package supports only pgx/v3. I can help with adding v4 (keeping v3 functional) if you're interested. This PR is the first small step toward v4 - it replaces github.com/jackc/pgx/pgtype that used to be subpackage in v3 with github.com/jackc/pgtype that is the same package but extracted to standalone repo in v4.

PS: what do you think about adding linting and tests to PRs using Github Actions? I can help with this as well if you do not have time to take care of the project.

[bgentry]

Hi @vgarvardt, I recommend you check out my new project, River. River already does quite a bit more than que-go, uses a much more scalable design, and is designed for modern Go and pgx/v5.

I will be archiving this repository shortly.