"Stop Execution" button and notebookInterruptibleKernel key don't work

[roblourens]

We have a button in core which is essentially the same as the Interrupt button, but it never appears because a context key is not updated for kernels that implement interrupt. Not sure what the history is here.

[vscode-triage-bot]

Pinging notebook topic followers: @claudiaregio @rchiodo @greazer @DonJayamanne @jilljac @IanMatthewHuff @tanhakabir @dynamicwebpaige

[greazer]

Be sure to fix microsoft/vscode-jupyter#7661 at the same time.