Most arguments to open_dataset should be keyword only

[shoyer]

open_dataset has a long list of arguments: xarray.open_dataset(filename_or_obj, group=None, decode_cf=True, mask_and_scale=None, decode_times=True, autoclose=None, concat_characters=True, decode_coords=True, engine=None, chunks=None, lock=None, cache=None, drop_variables=None, backend_kwargs=None, use_cftime=None)

Similarly to the case for pandas (pandas-dev/pandas#27544), it would be nice to make most of these arguments keyword-only, e.g., def open_dataset(filename_or_obj, group, *, ...). For consistency, this would also apply to open_dataarray, decode_cf, open_mfdataset, etc.

This would encourage writing readable code when calling open_dataset() and would allow us to use better organization when adding new arguments (e.g., decode_timedelta in #4071).

To make this change, we could make use of the deprecate_nonkeyword_arguments decorator from pandas-dev/pandas#27573

[mathause]

Now implemented with #4989