notebook variables view should handle very long lists

[amunger]

viewing a very long list in the variables viewer will not show all entries.

aLongList = []
for i in range(1000000):
    aLongList.append('listEntry' + str(i))

debug variables view handles this with nested groups:

[amunger]

verification steps:

1. enable the notebook variable view with "notebook.experimental.variablesView": true
2. run the cell

aLongList = []
for i in range(1000000):
    aLongList.append('listEntry' + str(i))

3. open the variable view in the debug sideview
4. spot check that the children of the aLongList variable are accurate