Optimize parse parts

[p1c2u]

This pull request refactors the parse_parts function in pathable/parsers.py to simplify its logic and improve readability. The main change is the removal of the nested append_split helper function, with its logic now inlined directly into the main loop. This results in a more straightforward and easier-to-follow function.

Refactoring and code simplification:

• Inlined the logic of the append_split helper function directly into the main loop, removing unnecessary function indirection and making the handling of str and bytes parts more explicit and readable.
• Removed the reversal of parts processing order and now processes parts in their original order, which may affect the output order if the function is used elsewhere.
• Improved handling of str and bytes types by directly checking for empty strings and separator presence, reducing code complexity.