Implement Cell Debugging - Notebook Editor

[IanMatthewHuff]

Breakdown of approximate timing for our native debugging spec

[rchiodo]

Two different options here

1. Use VS code's debugger
   • Spike to prevent VSC from opening a document (skip frame, etc)
   • DAP in the middle (3d)
2. Use the DAP ourselves and talk to the debugger/handle all of the messages ourselves.
   • Extension as debug controller
   • Variable explorer (1d)
   • Debugger owns variable eval (1w)
   • Custom toolbar 1d

Stuff for both
* Call DAP on PTVSD (2w)
* Margins (1w)

[jmew]

https://github.com/microsoft/vscode-python/issues/7913

[DavidKutu]

dupe of #1652