WIP [ArrayManager] API: setitem to set new columns / loc+iloc to update inplace
xref indexing work item of #39146

This actually started as trying to get the tests passing for `tests/frame/indexing`, but while doing that I needed to make decisions on what the behaviour should be for setitem / loc / iloc assignment (so related other ongoing discussions about this). 

Very much a draft, but a good way to see what we actually want / the consequences of those choices.

cc @jbrockmendel 