Support source position annotations

@LuoZijun made a create for attaching source position information to an error here: https://github.com/LuoZijun/oioioi

Someone on reddit wanted this, and I think it's a feature that maybe should be incorporated into `oi`.

One way to do this would be to add an `err_at_position(location, position)`, which you would have to call as `err_at_position(location, here!())`. Unfortunately, since the position information is generated via a macro, the user would have to supply the position at the call site.

The issue names no files or tests. Start by reviewing the linked LuoZijun/oioioi implementation and oi's existing error API, then determine how source positions should be represented and exposed. Done should include a settled interface for attaching a location and call-site position, such as err_at_position(location, here!()).

Written by the indexing model from the issue text.