ENH: Support scalar condition in `Series.where` and `DataFrame.where`
- [x] closes #53903
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
    - added/updated tests in `series/indexing/test_where.py`, `series/indexing/test_where.py`, `series/indexing/test_mask.py`
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [x] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
  - Regarding documentation, I am not sure anything needs to be updated since both Series.where and DataFrame.where specify array-like as a valid input type, and scalars are considered array-like.
- [x] Added an entry in the latest `doc/source/whatsnew/v2.1.0.rst` file if fixing a bug or adding a new feature.

@jbrockmendel Any other objections? > Any other objections?

Not about the implementation, I'm just not sure I want to support this.  In general we're trying to move towards being stricter in signatures.  But not strong enough an opinion to cause a stink here. Sounds like this feature needs more discussion and buy in in the original issue so going to close for now awaiting that decision. Can reopen once that has been done