allow int as a cellpath for select (#10048)

Description

This PR allows ints to be used as cell paths.

### Before
```nushell
❯ let index = 0
❯ locations | select $index
Error: nu:🐚:cant_convert

  × Can't convert to cell path.
   ╭─[entry #26:1:1]
 1 │ locations | select $index
   ·                    ───┬──
   ·                       ╰── can't convert int to cell path
   ╰────
```

### After
```nushell
❯ let index = 0
❯ locations | select $index
╭#┬───────location────────┬city_column┬state_column┬country_column┬lat_column┬lon_column╮
│0│http://ip-api.com/json/│city       │region      │countryCode   │lat       │lon       │
╰─┴───────────────────────┴───────────┴────────────┴──────────────┴──────────┴──────────╯
```
# User-Facing Changes
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# Tests + Formatting
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crates/nu-command/src/filters/select.rs

@@ -64,18 +64,20 @@ produce a table, a list will produce a list, and a record will produce a record.
         let columns: Vec<Value> = call.rest(engine_state, stack, 0)?;
         let mut new_columns: Vec<CellPath> = vec![];
         for col_val in columns {
+            let col_span = &col_val.span()?;
             match col_val {
                 Value::CellPath { val, .. } => {
                     new_columns.push(val);
                 }
                 Value::List { vals, .. } => {
-                    for val in vals {
-                        match val {
+                    for value in vals {
+                        let val_span = &value.span()?;
+                        match value {
                             Value::String { val, .. } => {
                                 let cv = CellPath {
                                     members: vec![PathMember::String {
                                         val: val.clone(),
-                                        span: Span::unknown(),
+                                        span: *val_span,
                                         optional: false,
                                     }],
                                 };
@@ -85,7 +87,7 @@ produce a table, a list will produce a list, and a record will produce a record.
                                 let cv = CellPath {
                                     members: vec![PathMember::Int {
                                         val: val as usize,
-                                        span: Span::unknown(),
+                                        span: *val_span,
                                         optional: false,
                                     }],
                                 };
@@ -106,7 +108,17 @@ produce a table, a list will produce a list, and a record will produce a record.
                     let cv = CellPath {
                         members: vec![PathMember::String {
                             val: val.clone(),
-                            span: Span::unknown(),
+                            span: *col_span,
+                            optional: false,
+                        }],
+                    };
+                    new_columns.push(cv.clone());
+                }
+                Value::Int { val, .. } => {
+                    let cv = CellPath {
+                        members: vec![PathMember::Int {
+                            val: val as usize,
+                            span: *col_span,
                             optional: false,
                         }],
                     };

crates/nu-command/tests/commands/select.rs

@@ -264,3 +264,11 @@ fn select_rows_with_variable_list() {
 
     assert_eq!(actual.out, "[[a, b, c]; [1, 2, 3], [7, 8, 9]]");
 }
+
+#[test]
+fn select_single_row_with_variable() {
+    let actual = nu!("let idx = 2;[{a: 1, b: 2} {a: 3, b: 5} {a: 3}] | select $idx | to nuon");
+
+    assert_eq!(actual.out, "[[a]; [3]]".to_string());
+    assert!(actual.err.is_empty());
+}