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/builtins/impure.rs

@@ -63,15 +63,16 @@ pub fn builtins_import(
             let parsed = rnix::ast::Root::parse(&contents);
             let errors = parsed.errors();
 
+            let file = source.add_file(path.to_string_lossy().to_string(), contents);
+
             if !errors.is_empty() {
                 return Err(ErrorKind::ImportParseError {
                     path,
+                    file,
                     errors: errors.to_vec(),
                 });
             }
 
-            let file = source.add_file(path.to_string_lossy().to_string(), contents);
-
             let result = crate::compile(
                 &parsed.tree().expr().unwrap(),
                 Some(path.clone()),