Provide the ability to split strings in columns via polars str-split (#14723)

# Description
Provides the ability to split string columns. This will change the
column type to list<str>.

```nushell
> ❯ : [[a]; ["one,two,three"]] | polars into-df | polars select (polars col a | polars str-split ",") | polars collect
╭───┬───────────────╮
│ # │       a       │
├───┼───────────────┤
│ 0 │ ╭───┬───────╮ │
│   │ │ 0 │ one   │ │
│   │ │ 1 │ two   │ │
│   │ │ 2 │ three │ │
│   │ ╰───┴───────╯ │
╰───┴───────────────╯

> ❯ : [[a]; ["one,two,three"]] | polars into-df | polars select (polars col a | polars str-split ",") | polars schema
╭───┬───────────╮
│ a │ list<str> │
╰───┴───────────╯
```



# User-Facing Changes
- Introduces new command `polars str-split`

crates/nu_plugin_polars/src/dataframe/command/string/mod.rs

@@ -5,6 +5,7 @@ mod replace_all;
 mod str_join;
 mod str_lengths;
 mod str_slice;
+mod str_split;
 mod to_lowercase;
 mod to_uppercase;
 
@@ -27,6 +28,7 @@ pub(crate) fn string_commands() -> Vec<Box<dyn PluginCommand<Plugin = PolarsPlug
         Box::new(Contains),
         Box::new(Replace),
         Box::new(ReplaceAll),
+        Box::new(str_split::StrSplit),
         Box::new(StrJoin),
         Box::new(StrLengths),
         Box::new(StrSlice),

crates/nu_plugin_polars/src/dataframe/command/string/str_split.rs

@@ -0,0 +1,92 @@
+use crate::{
+    values::{CustomValueSupport, NuDataFrame, NuExpression},
+    PolarsPlugin,
+};
+
+use nu_plugin::{EngineInterface, EvaluatedCall, PluginCommand};
+use nu_protocol::{
+    Category, Example, LabeledError, PipelineData, Signature, Span, Spanned, SyntaxShape, Type,
+    Value,
+};
+use polars::df;
+
+#[derive(Clone)]
+pub struct StrSplit;
+
+impl PluginCommand for StrSplit {
+    type Plugin = PolarsPlugin;
+
+    fn name(&self) -> &str {
+        "polars str-split"
+    }
+
+    fn description(&self) -> &str {
+        "Split the string by a substring. The resulting dtype is list<str>."
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build(self.name())
+            .required("expr", SyntaxShape::Any, "Separator expression")
+            .input_output_types(vec![(
+                Type::Custom("expression".into()),
+                Type::Custom("expression".into()),
+            )])
+            .category(Category::Custom("dataframe".into()))
+    }
+
+    fn examples(&self) -> Vec<Example> {
+        vec![Example {
+            description: "Split the string by comma, then create a new row for each string",
+            example: r#"[[a]; ["one,two,three"]] | polars into-df 
+                | polars select (polars col a | polars str-split "," | polars explode) 
+                | polars collect"#,
+            result: Some(
+                NuDataFrame::from(
+                    df!(
+                    "a" => ["one", "two", "three"]
+                    )
+                    .expect("Should be able to create a dataframe"),
+                )
+                .into_value(Span::test_data()),
+            ),
+        }]
+    }
+
+    fn run(
+        &self,
+        plugin: &Self::Plugin,
+        engine: &EngineInterface,
+        call: &EvaluatedCall,
+        input: PipelineData,
+    ) -> Result<PipelineData, LabeledError> {
+        let separator = call.req::<Spanned<Value>>(0).and_then(|sep| {
+            let sep_expr = NuExpression::try_from_value(plugin, &sep.item)?;
+            Ok(Spanned {
+                item: sep_expr,
+                span: sep.span,
+            })
+        })?;
+
+        let expr = NuExpression::try_from_pipeline(plugin, input, call.head)?;
+        let res: NuExpression = expr
+            .into_polars()
+            .str()
+            .split(separator.item.into_polars())
+            .into();
+        res.to_pipeline_data(plugin, engine, call.head)
+            .map_err(LabeledError::from)
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use nu_protocol::ShellError;
+
+    use super::*;
+    use crate::test::test_polars_plugin_command;
+
+    #[test]
+    fn test_examples() -> Result<(), ShellError> {
+        test_polars_plugin_command(&StrSplit)
+    }
+}