fix(parser): span of $it/$in set to the first character of its scope (#14817)

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Follow-up PR of #14789 
The span of `$it/$in` is set to a 0-width one with start/end pointing to
the start of its scope, mainly for error messages positioning.

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crates/nu-parser/src/parser.rs

@@ -3435,7 +3435,8 @@ 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"$it".to_vec(), Span::unknown(), Type::Any, false);
+    let pos = spans.first().map(|s| s.start).unwrap_or(0);
+    let var_id = working_set.add_variable(b"$it".to_vec(), Span::new(pos, pos), Type::Any, false);
     let expression = parse_math_expression(working_set, spans, Some(var_id));
     let span = Span::concat(spans);
 
@@ -6633,7 +6634,12 @@ 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"$in".into(), Span::unknown(), Type::Any, false);
+    let var_id = working_set.add_variable(
+        b"$in".into(),
+        Span::new(span.start, span.start),
+        Type::Any,
+        false,
+    );
     let mut expr = expr.clone();
     expr.replace_in_variable(working_set, var_id);