feat(tvix/eval): fancy-format parse errors returned by rnix

This change is quite verbose, so a little bit of explaining:

1. To correctly format parse errors, errors must be able to return
   more than one annotated span (the parser returns a list of errors
   for each span).

   To accomplish this, the structure of how the `Diagnostic` struct
   which formats an error is constructed has changed to delegate the
   creation of the `SpanLabel` vector to the kind of error.

2. The rnix structures don't have human-readable output formats by
   default, so some verbose methods for formatting them in
   human-readable ways have been added in the errors module. We might
   want to move these out into a submodule.

3. In many cases, the errors returned by rnix are a bit strange - so
   while we format them with all information that is easily available
   they may look weird or not necessarily help users. Consider this CL
   only a first step in the right direction.

Change-Id: Ie7dd74751af9e7ecb35d751f8b087aae5ae6e2e8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6871
Reviewed-by: sterni <sternenseemann@systemli.org>
Autosubmit: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/eval.rs

@@ -39,13 +39,12 @@ pub fn interpret(code: &str, location: Option<PathBuf>, options: Options) -> Eva
     let errors = parsed.errors();
 
     if !errors.is_empty() {
-        for err in errors {
-            eprintln!("parse error: {}", err);
-        }
-        return Err(Error {
+        let err = Error {
             kind: ErrorKind::ParseErrors(errors.to_vec()),
             span: file.span,
-        });
+        };
+        err.fancy_format_stderr(&source);
+        return Err(err);
     }
 
     // If we've reached this point, there are no errors.