refactor: more error info, span merge

[m-span]

• merges ast into parser moves error from span to parser
• merges span, error objects into single, custom objects
• handles error struct conversion better so errors are more informative
• fixes existing bad error message for interpolated strings
• adds new bad error message for interpolated strings

[max-sixty]

Nice, great to see you've made progress!

It would be easier to review if we can clean up some of the diffs — looks like we have some merge conflicts and some formatting-only changes. (I don't think we need to be sticklers about splitting things into commits within a PR.)

It also would be easier to review if we can split things into smaller PRs; for example there's a nice -v option that we could merge immediately. Review difficulty seems to increase super-linearly with size, so breaking things up is easier. But it's also not fun to do ex-post, so it may not be worthwhile at this stage.

Does the prqlc-ast / prqlc-parser crate merging give us anything at the moment? I'm hesitant to go through another refactor without either a clear reduction in code complexity or a win on language functionality...

Nice work on the error message improvements — that part sounds great.

[m-span]

Dupilicate code and concepts in span and error were merged. I moved ast files back to prqlc-ast, except for error, which had to stay in the parser to avoid circular deps.

[max-sixty]

Superb, thanks!