Explorer: support exploring the variables of an xarray Dataset

[maximlt]

Reviewing https://github.com/holoviz/hvplot/pull/1256 made me realize that the explorer doesn't support selecting the variables of an xarray Dataset, it seems to be picking up the first variable available (air in the example below).

import hvplot.xarray
import xarray as xr

ds_at = xr.tutorial.open_dataset('air_temperature')
ds_at['air2'] = ds_at.air
ds_at.hvplot.explorer()