ENH: Add support for more placeholders in `guess_datetime_format`
Add support for day of week and meridiem placeholders and any
combination of placeholders supported by `strftime` that do not
correspond to a datetime attribute.

- [x] closes #43901
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [x] whatsnew entry

I'm not sure if this fix actually qualifies as a bug fix and not and enhancement.  Before, `guess_datetime_format` would output a somewhat invalid format string that included literal content from the parsed datetime such as "Tue" or "AM".
