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".

@jreback @mroeschke Made the requested changes.  Went ahead and squashed everything to have just one commit.  Though I'm not sure if reviewers typically prefer to have the commit history of the PR until final approval. @jreback Anything else needed for this?  Not sure what happened with the cancelled github action. @davesque sorry for the delay. Could you merge in master one more time, and then this should be good to merge. Thanks @davesque 

(May be a performance improvement for all formats according to https://github.com/pandas-dev/pandas/pull/43900#discussion_r724457595)