Add split cell-path (#13705)

this PR should close #12168

# Description
Add `split cell-path`, inverse of `into cell-path`.

# User-Facing Changes
Currently there is no way to make use of cell-path values as a user,
other than passing them to builtin commands. This PR makes more use
cases possible.

crates/nu-command/src/default_context.rs

@@ -326,6 +326,7 @@ pub fn add_shell_command_context(mut engine_state: EngineState) -> EngineState {
             IntoString,
             IntoGlob,
             IntoValue,
+            SplitCellPath,
         };
 
         // Env