Refactor high level folders

[rchiodo]

This is a rollup issue of the following issues:

• Refactor kernel related code into a 'Kernel' subfolder off of 'src' #8970
• Refactor notebook related code into a 'Notebooks' folder off of src #8971
• Refactor 'Interactive Window' related code into an 'InteractiveWindow' folder #8972
• Refactor 'language services' related code into a 'LanguageServices' folder #8973
• Refactor standalone UI parts into a 'Webviews' folder #8974
• Refactor test code to go under a single 'test' folder #8975
• Refactor extension initialization code into an 'Extension' subfolder off of src #8981
• Eliminate ICell usage everywhere and use nbformat.ICell or NotebookCell #9324?
• Refactor src\ipywidgets out of source folder. It could be an npm package #9337

Once all of those issues are complete we should have:

• src and test at the root
• src should have:
  • Kernels
  • Notebooks
  • InteractiveWindow
  • LanguageServices
  • Webviews
  • Test

[amunger]

I would suggest an Extension top level folder as well: #8981

[rebornix]

@amunger can that be single file extension.ts w/ or w/o a few helper files? I feel like a single entry file for the extension would be easier to understand.

[amunger]

Yeah I think a single extension.ts starting point would be great to have since it follows the convention used in the extension documentation