feat(polars): add pow (**) operator for polars expressions (#15598)

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# Description
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This PR adds the exponent operator ("**") to polars expressions.

```nushell
  > [[a b]; [6 2] [4 2] [2 2]] | polars into-df | polars select a b {c: ((polars col a) ** 2)}
  ╭───┬───┬───┬────╮
  │ # │ a │ b │ c  │
  ├───┼───┼───┼────┤
  │ 0 │ 6 │ 2 │ 36 │
  │ 1 │ 4 │ 2 │ 16 │
  │ 2 │ 2 │ 2 │  4 │
  ╰───┴───┴───┴────╯
```

# User-Facing Changes
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helps us keep track of breaking changes. -->
No breaking changes. Users are enabled to use the `**` operator in
polars expressions.

# Tests + Formatting
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An example in `polars select` was modified to showcase the `**`
operator.

# After Submitting
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crates/nu_plugin_polars/src/dataframe/command/data/select.rs

@@ -72,7 +72,7 @@ impl PluginCommand for LazySelect {
             },
             Example {
                 description: "Select a column from a dataframe using a mix of expressions and record of expressions",
-                example: "[[a b]; [6 2] [4 2] [2 2]] | polars into-df | polars select a b {c: ((polars col a) * 2)}",
+                example: "[[a b]; [6 2] [4 2] [2 2]] | polars into-df | polars select a b {c: ((polars col a) ** 2)}",
                 result: Some(
                     NuDataFrame::try_from_columns(
                         vec![
@@ -84,7 +84,7 @@ impl PluginCommand for LazySelect {
                             vec![Value::test_int(2), Value::test_int(2), Value::test_int(2)]),
                             Column::new(
                             "c".to_string(),
-                            vec![Value::test_int(12), Value::test_int(8), Value::test_int(4)])
+                            vec![Value::test_int(36), Value::test_int(16), Value::test_int(4)])
                         ],
                         None,
                     )

crates/nu_plugin_polars/src/dataframe/values/nu_expression/custom_value.rs

@@ -103,6 +103,9 @@ fn with_operator(
         Operator::Math(Math::FloorDivide) => {
             apply_arithmetic(plugin, engine, left, right, lhs_span, Div::div)
         }
+        Operator::Math(Math::Pow) => {
+            apply_arithmetic(plugin, engine, left, right, lhs_span, Expr::pow)
+        }
         Operator::Comparison(Comparison::Equal) => Ok(left
             .clone()
             .apply_with_expr(right.clone(), Expr::eq)