expose a uriUserInfo parser

There is not currently any way to split the uriUserInfo into user name and password, which leaves anyone who needs to do that to implementing part of URI parsing themselves.

It's not quite as simple as just `break (==':')`; as seen in [defaultUserInfoMap](https://hackage.haskell.org/package/network-uri-2.6.4.2/docs/src/Network.URI.html#defaultUserInfoMap) the trailing '@' complicates things.

Start with the existing uriUserInfo handling and inspect defaultUserInfoMap in Network.URI, linked from the issue. Trace how the trailing '@' is handled before defining the parser's behavior. Done means callers can obtain the user name and password without reimplementing URI parsing, with coverage for the delimiter edge case.

Written by the indexing model from the issue text.