API: mode.nan_is_na to consistently distinguish NaN-vs-NA

[jbrockmendel]

• closes #xxxx (Replace xxxx with the GitHub issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

As discussed on the last dev call, this implements "mode.nan_is_na" (default True) to consider NaN as either always-equivalent or never-equivalent to NA.

This sits on top of

• TST: nan->NA in non-construction tests #62021, which trims the diff here by updating some tests to use NA instead of NaN.
• API: consistent NaN treatment for pyarrow dtypes #61732 which implements the option but only for pyarrow dtypes.
• API: improve dtype in df.where with EA other #62038 which addresses an issue in DataFrame.where

Still need to

• Add docs for the new option, including whatsnew section
• deal with a kludge in algorithms.rank; fixed by API: rank with nullable dtypes preserve NA #62043
• deal with a kludge in read_csv with engine="pyarrow"
• Add tests for the issues this addresses