show dimension coordinates at top of coordinates repr

[dcherian]

Is your feature request related to a problem? Please describe.
I have datasets with lots of non-dim coord variables. Its annoying to search through and look at the dimension coordinates to get an idea of what subset of data I am looking at.

Describe the solution you'd like
I think we should show dimension coordinate variables at the top of the coordinates repr.

Example code

ds = xr.Dataset()
ds.coords["as"] = 10
ds["var"] = xr.DataArray(np.ones((10,)), dims="x", coords={"x": np.arange(10)})
ds

Related #4409

[dcherian]

Closing as dupe of #712