Adopt notebook/toolbar contribution point for native notebooks

[joyceerhl]

Just had a chat with @rebornix about this. With Sunday night's Insiders build, extensions will be able to contribute items to notebook/toolbar, with the following groups:

• navigation/edit
• navigation/execute <-- This will house restart kernel and interrupt kernel
• navigation <-- This will house other actions like open variable view and export
• status <-- This appears on the right of the notebook toolbar and currently houses only the kernel picker. In future we can consider moving the local/remote status item up here, but for now we should not contribute to this group

We will be able to specify a shortTitle for each command that appears in the notebook toolbar. When present, this shortTitle attribute will be displayed next to the command's associated codicon in the toolbar, so as not to take up too much horizontal space with a lengthier title. VS Code will then pick either shortTitle or the existing title to display depending on context.

[claudiaregio]

Wahoooo ok entry points to move into the notebook toolbar:

1. Short Title: Restart , Title: Restart Kernel - Navigation/Execute
2. Short Title: Interrupt, Title: Interrupt Kernel - Navigation/Execute
3. Short Title: Variables, Title: Open Variable View - Navigation
4. Short Title: Export, Title: Export As - Navigation

[joyceerhl]

PR has been merged, should go out in tonight's Jupyter extension insiders build.

[IanMatthewHuff]

Validated.