Deprecate split-by command (#14019)

# Description
I'm not quite sure what the point of the `split-by` command is. The only
example for the command seems to suggest it's an additional grouping
command. I.e., a record that seems to be the output of the `group-by`
command is passed to `split-by` which then adds an additional layer of
grouping based on a different column.

# User-Facing Changes
Breaking change, deprecated the command.

crates/nu-command/src/filters/split_by.rs

@@ -1,5 +1,6 @@
 use indexmap::IndexMap;
 use nu_engine::command_prelude::*;
+use nu_protocol::report_shell_warning;
 
 #[derive(Clone)]
 pub struct SplitBy;
@@ -27,6 +28,15 @@ impl Command for SplitBy {
         call: &Call,
         input: PipelineData,
     ) -> Result<PipelineData, ShellError> {
+        report_shell_warning(
+            engine_state,
+            &ShellError::Deprecated {
+                deprecated: "The `split_by` command",
+                suggestion: "Please use the `group-by` command instead.",
+                span: call.head,
+                help: None,
+            },
+        );
         split_by(engine_state, stack, call, input)
     }