cftime_range missing freq ("W") - Invalid frequency string provided

Is your feature request related to a problem?

I tend to use xr.cftime_range as a drop in for pandas.date_range. My only reason here is I may able to shave a line and not import pandas.

In doing xr.cftime_range("2020-01-01", periods=1, freq="W") I get

AttributeError                            Traceback (most recent call last)
/opt/userenvs/ray.bell/main/lib/python3.9/site-packages/xarray/coding/cftime_offsets.py in to_offset(freq)
    729         try:
--> 730             freq_data = re.match(_PATTERN, freq).groupdict()
    731         except AttributeError:

AttributeError: 'NoneType' object has no attribute 'groupdict'

During handling of the above exception, another exception occurred:

ValueError                                Traceback (most recent call last)
/tmp/ipykernel_549/3579801314.py in <module>
----> 1 xr.cftime_range("2020-01-01", periods=1, freq="W")

/opt/userenvs/ray.bell/main/lib/python3.9/site-packages/xarray/coding/cftime_offsets.py in cftime_range(start, end, periods, freq, normalize, name, closed, calendar)
   1041         dates = _generate_linear_range(start, end, periods)
   1042     else:
-> 1043         offset = to_offset(freq)
   1044         dates = np.array(list(_generate_range(start, end, periods, offset)))
   1045 

/opt/userenvs/ray.bell/main/lib/python3.9/site-packages/xarray/coding/cftime_offsets.py in to_offset(freq)
    730             freq_data = re.match(_PATTERN, freq).groupdict()
    731         except AttributeError:
--> 732             raise ValueError("Invalid frequency string provided")
    733 
    734     freq = freq_data["freq"]

ValueError: Invalid frequency string provided

In pandas I get

>>> pd.date_range("2020-01-01", periods=1, freq="W")
DatetimeIndex(['2020-01-05'], dtype='datetime64[ns]', freq='W-SUN')

Describe the solution you'd like

No response

Describe alternatives you've considered

Closing this issue as outside of scope

Additional context

No response

Reproduce the reported call, then read xarray/coding/cftime_offsets.py around cftime_range and to_offset, where the traceback identifies the failure. Check the existing offset parsing and nearby tests; done means a W frequency is accepted and produces the pandas-compatible weekly result without breaking existing frequencies.