wip: depr partial row-wise multi-column-wise enlargement

[MarcoGorelli]

• 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.

just curious to see what fails, and I'm questioning whether it's worth adding complexity to support this

Is anyone particularly attached to this? If not, I'd suggest deprecating, because:

• the workaround on the users' side is very simple
• __setitem__ is already incredibly complex and spaghettified
• there's only 3 tests which use this pattern
• there's little time left until 3.0, and I don't have time to work on this further before 2.2

NOTE: I'm not actually sure that disallowing this is even the way to go, will sleep on it

[MarcoGorelli]

actually addressed this in the end, so closing