Fix highlighting of spread subexpressions in records (nushell#11202)

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# Description
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It turns out that I left a bug in
[nushell#11144](nushell#11144), which
introduced a spread operator in record literals. When highlighting
subexpressions that are spread inside records, the spread operator and
the token before it are insert twice. Currently, when you type `{ ...()
}`, this is what you'll see:


![image](https://github.com/nushell/nushell/assets/45539777/9a76647a-6bbe-426e-95bc-50becf2fa537)

With the PR, the behavior is as expected:


![image](https://github.com/nushell/nushell/assets/45539777/36bdab23-3252-4500-8317-51278da0e869)

I'm still not sure how `FlatShape` works, I just copied the existing
logic for flattening key-value pairs in records, so it's possible
there's still issues, but I haven't found any yet (tried spreading
subexpressions, variables, and records).

# User-Facing Changes
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Highlighting for subexpressions spread inside records should no longer
be screwed up.

# Tests + Formatting
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Is there any way to test flattening/syntax highlighting?

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crates/nu-parser/src/flatten.rs

@@ -443,8 +443,15 @@ pub fn flatten_expression(
                             output.push((Span::new(last_end, op_span.start), FlatShape::Record));
                         }
                         output.push((*op_span, FlatShape::Operator));
+                        last_end = op_span.end;
 
                         let flattened_inner = flatten_expression(working_set, record);
+                        if let Some(first) = flattened_inner.first() {
+                            if first.0.start > last_end {
+                                output
+                                    .push((Span::new(last_end, first.0.start), FlatShape::Record));
+                            }
+                        }
                         if let Some(last) = flattened_inner.last() {
                             last_end = last.0.end;
                         }