CoW warning mode: warn in case of chained assignment
Follow-up PR on https://github.com/pandas-dev/pandas/pull/55428 (only last commit is relevant here, will be reviewable once parent PR is merged), xref https://github.com/pandas-dev/pandas/issues/56019

Adding warnings for the case of chained assignment, i.e. cases for which we will issue the ChainedAssignmentError warning in the future with CoW enabled. But since this is the main breaking change of CoW, important to have a clear warning as well in the warn mode, and it is enabled in the default mode as well.

xref https://github.com/pandas-dev/pandas/issues/56019
As for the other PR, I would warn in the default mode as well. This will need a similar system as for the other chained PRs that checks the cacher (that said, I found a bug in this mode that I am fixing now) thx @jorisvandenbossche 