Context around error diagnostics

[goto-bus-stop]

We moved from miette to ariadne so we can create diagnostics that reference multiple files at once. This caused one major regression in our diagnostic display: ariadne often prints way less context. A typical example:

Before	After

In the After case it's very hard to see where exactly this is happening, you basically have to jump to the given line number in your editor to make sense of it. Having a few lines displayed around the start and end of each labeled span would help a lot. I think this is not (yet) possible with ariadne, so we should contribute that feature if the author wants it.