wip: depr partial row-wise multi-column-wise enlargement
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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
actually addressed this in the end, so closing