REF: Improve Situation around `rule_code` for DateOffsets

[CloseChoice]

Following the discussions in this PR, there are a couple of things that should be looked into, before changing rule_code:

1. One thing that came to my mind is to add rule_code for DateOffsets. This would help us get rid of the except part in this PR
2. Refactor parse_date_string to not need this information at all. As you mentioned REGR: NotImplementedError: Prefix not defined when slicing offset with loc #47547 (comment) freq is actually used in very limited cases (as I can see, it only used in the chain parse_time_string -> parse_datetime_string_with_reso -> _parse_dateabbr_string -> quarter_to_myear -> get_rule_month and in _parse_dateabbr_string where it is checked against M. So there might be a good way of working around this by finding the rule_month (as given by get_rule_month) and checking for M first and implementing a special case for this somewhere not so deep down in the chain.

There are also a couple todos written in the code, so I think we could improve here quite some things.