add drop_existing kwarg to set_xindex

[ianhi]

• Closes Add new kwarg drop_existing to set_xindex #11007
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• [NA] New functions/methods are listed in api.rst

[benbovy]

Thanks @ianhi, I think that this is a convenient addition!

I'd just rename drop_existing to simply drop. The latter is a bit less self-explanatory but more consistent with the rest of the Xarray API where drop is already a parameter of many methods like .reset_index, .reset_coords, .isel, .where, etc.