chore: add codeowners file

[hjr3]

This makes it clear who is maintaining which package. This will also allow commit privileges for a specific package instead of the entire repo.

[hjr3]

From https://github.com/brianc/node-postgres/pull/2105#issuecomment-589179652, I was maintaining pg-connection-string but my commit access has been removed now that it is in the monorepo.

@brianc what do you think about this approach?

[brianc]

it makes sense to me - sorry the build was broken. might wanna rebase on it!

[hjr3]

@brianc all good now