add is_const to help commands and scope commands (#14125)

# Description

This PR adds `is_const` to `help commands` and `scope commands` so we
can see which commands are const commands.


![image](https://github.com/user-attachments/assets/f2269f9d-5042-40e4-b506-34d69096fcd1)

crates/nu-command/src/help/help_commands.rs

@@ -220,6 +220,7 @@ fn build_help_commands(engine_state: &EngineState, span: Span) -> Vec<Value> {
             "params" => param_table,
             "input_output" => input_output_table,
             "search_terms" => Value::string(search_terms.join(", "), span),
+            "is_const" => Value::bool(decl.is_const(), span),
         };
 
         found_cmds_vec.push(Value::record(record, span));