allow != for polars (#14263)

# Description

This PR fixes a problem where not equal in polars wasn't working with
strings.

## Before
```nushell
let a = ls | polars into-df
$a.type != "dir"
Error: nu:🐚:type_mismatch

  × Type mismatch during operation.
   ╭─[entry #16:1:1]
 1 │ $a.type != "dir"
   · ─┬      ─┬ ──┬──
   ·  │       │   ╰── string
   ·  │       ╰── type mismatch for operator
   ·  ╰── NuDataFrame
   ╰────
```

## After
```nushell
let a = ls | polars into-df
$a.type != "dir"
╭──#──┬─type──╮
│ 0   │ false │
│ 1   │ false │
│ 2   │ false │
...
```

/cc @ayax79 to make sure I did this right.

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crates/nu_plugin_polars/src/dataframe/values/nu_dataframe/between_values.rs

@@ -334,6 +334,10 @@ pub(super) fn compute_series_single_value(
             Value::Float { val, .. } => {
                 compare_series_float(&lhs, *val, ChunkedArray::not_equal, lhs_span)
             }
+            Value::String { val, .. } => {
+                let equal_pattern = format!("^{}$", fancy_regex::escape(val));
+                contains_series_pat(&lhs, &equal_pattern, lhs_span)
+            }
             Value::Date { val, .. } => compare_series_i64(
                 &lhs,
                 val.timestamp_millis(),