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       │
╰─┴───────────────────────┴───────────┴────────────┴──────────────┴──────────┴──────────╯
```
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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());
+}