[REFACTOR] Split up `exec_pipeline()` for readability

[itislu]

The function was very dense and we resorted to bad practices like the comma operator to keep the function length within 25 lines for Norminette.

• Split up long else block of exec_pipeline() into separate exec_cmd_table() function and rename the original function to iter_pipeline().
• Change unnecessary long while (check cmd_table_type) loop check to while (true) + if (check cmd_table_type) + break.
• Remove comma operator.