fix: unknown span for special variables $in/$it (#14789)

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# Description
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This PR addresses the issue of inconsistent spans of special variables
of `$in/$in`, as discussed in
https://github.com/nushell/nushell/pull/14770#discussion_r1908729364.

Instead of making the `declaration_span` to be Option, which will cause
too many changes that we may want to avoid, this PR set the spans to be
`unknown`.


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No

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crates/nu-parser/src/parser.rs

@@ -3435,7 +3435,7 @@ pub fn parse_full_signature(working_set: &mut StateWorkingSet, spans: &[Span]) -
 }
 
 pub fn parse_row_condition(working_set: &mut StateWorkingSet, spans: &[Span]) -> Expression {
-    let var_id = working_set.add_variable(b"$it".to_vec(), Span::concat(spans), Type::Any, false);
+    let var_id = working_set.add_variable(b"$it".to_vec(), Span::unknown(), Type::Any, false);
     let expression = parse_math_expression(working_set, spans, Some(var_id));
     let span = Span::concat(spans);
 
@@ -6633,7 +6633,7 @@ fn wrap_expr_with_collect(working_set: &mut StateWorkingSet, expr: Expression) -
 
     // IN_VARIABLE_ID should get replaced with a unique variable, so that we don't have to
     // execute as a closure
-    let var_id = working_set.add_variable(b"$in".into(), expr.span, Type::Any, false);
+    let var_id = working_set.add_variable(b"$in".into(), Span::unknown(), Type::Any, false);
     let mut expr = expr.clone();
     expr.replace_in_variable(working_set, var_id);