Remove custom entry from Jupyter Notebook & Cell Metadata

[DonJayamanne]

This is an unnecessary layer
Everywhere in code we need to look at custom, when there's no such thing in Jupyter (specification) metdata.
This is a carry over from the old VS Code notebook Notebook format.
All metadata had to be stored under custom and this was never migrated when custom in core was removed.

This shows up in SCM view as well
I.e. when using the diff view, the custom node is displayed, and this is wrong, as there's no such thing in the spec.

Removing this will:

• Improve SCM
• Make it easier for extensions to access the metadata
• Ensure the API lines up with the Jupyter specification

Tasks

Beta Give feedback

1. Store isInteractiveWindowMessageCell in metadata instead of custom metadata vscode-jupyter#15378
   debt +1
   
2. Notebook Diff Viewer does not reflect true state of ipynb (JSON contents) #207968
   insiders-released notebook notebook-diff +3
   
3. Use built in ipynb serializer to convert NotebookData to ipynb vscode-jupyter#15380
   debt on-testplan +2
   
4. Future proof access to Notebook Metadata julia-vscode/julia-vscode#3568
   area-notebook +1
   
5. Timeline diffview for latest item is incorrect due to sorting of properties in metadata vscode-jupyter#15402
   bug on-testplan +2
   

[DonJayamanne]

@rebornix /cc
We will need to coordinate this with partner teams that read/write ipynb notebooks

[DonJayamanne]

@davidanthoff
From what I can tell, the only place where the Notebook or Cell metadata is accessed, is in the notebookFeature.ts file.
My plan is to submit a PR that would make a change to support both the old custom and the new format.
This way, it would be backwards compatible if and when we move away from the custom property.

Should be easy enough to create a single function that returns the metadata and use that everywhere.

How does that sound?

[DonJayamanne]

@colombod

Looking at the code in Polyglot Notebooks (Interactive)
I can see it is used in a large number of places notebookControllers.ts, metadataUtilities.ts, notebookSerializers.ts, etc.
I think the best and safest way to make such a change in your repo would be to have an if condition that conditionally uses custom based on the version of VS Code being used.
From what I can tell, tehre are a number of files that would be effected, hence the change would not be as simple/straightforward, happy to discuss offline.

[davidanthoff]

@DonJayamanne that definitely works!

[DonJayamanne]

Closing as done.