feat: allow for multiindex columns to be selected using nw.col

[MarcoGorelli]

I don't have time for a complete write-up right now, but basically, this: https://discord.com/channels/1235257048170762310/1235257049626181656/1287833882216894506

EDIT looks like we already support this:

In [22]: nw.from_native(df_mi).select(nw.col([(1, 4)])*2).to_native()
Out[22]:
    1
    4
c  14

In [23]: df_mi
Out[23]:
a  1  2  3
b  4  5  6
c  7  8  9

I'll add a note about this in the "pandas quirks" section fo the docs, but strongly recommend that library authors take the decision to only allow unique string names 😅 but if they really want multi-index column support, we at least offer them a way to do that, even if it's a little inconvenient