Support complex arrays in xr.corr

[headtr1ck]

• Closes xr.corr produces incorrect output for complex arrays #7340
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

[headtr1ck]

Unfortunately dask does not support conjugate yet and raises an error.

Not sure how to proceed...

[dcherian]

IT supports .conj so we now have a different error:

/home/runner/work/xarray/xarray/xarray/tests/test_computation.py:1597: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
/home/runner/work/xarray/xarray/xarray/core/computation.py:1300: in cov
    return _cov_corr(da_a, da_b, dim=dim, ddof=ddof, method="cov")
/home/runner/work/xarray/xarray/xarray/core/computation.py:1402: in _cov_corr
    valid_count = valid_values.sum(dim) - ddof
/home/runner/work/xarray/xarray/xarray/core/_aggregations.py:1855: in sum
    return self.reduce(
/home/runner/work/xarray/xarray/xarray/core/dataarray.py:3667: in reduce
    var = self.variable.reduce(func, dim, axis, keep_attrs, keepdims, **kwargs)
/home/runner/work/xarray/xarray/xarray/core/variable.py:1944: in reduce
    axis = self.get_axis_num(dim)
/home/runner/work/xarray/xarray/xarray/core/common.py:215: in get_axis_num
    return tuple(self._get_axis_num(d) for d in dim)
/home/runner/work/xarray/xarray/xarray/core/common.py:215: in <genexpr>
    return tuple(self._get_axis_num(d) for d in dim)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <xarray.Variable (x: 2, time: 2)>
array([[ True,  True],
       [ True,  True]])
dim = ['time', 'x']

    def _get_axis_num(self: Any, dim: Hashable) -> int:
        try:
            return self.dims.index(dim)
        except ValueError:
>           raise ValueError(f"{dim!r} not found in array dimensions {self.dims!r}")
E           ValueError: ['time', 'x'] not found in array dimensions ('x', 'time')

[dcherian]

LGTM thanks!