Support **kwargs form in .chunk()

[jbusecke]

Is your feature request related to a problem?

Take a simple example

import xarray as xr
da = xr.DataArray([1,2,4], dims=['x'])

If I want to chunk the array I can do:

da.chunk({'x':1})

but I cant do this:

da.chunk(x=1)

TypeError: chunk() got an unexpected keyword argument 'x'

Describe the solution you'd like

I would like to be able to use the .chunk() method for dataarrays in both ways, since it is common for many xarray methods to work either way (e.g. isel()/coarsen()/etc).

@TomNicholas

Describe alternatives you've considered

No response

Additional context

No response