Create meta context keys for notebook keybinding enablement

[joyceerhl]

The when clauses for notebook editor keybindings are getting too long to easily parse. Per @IanMatthewHuff we should create our own meta context keys so we can express the conditions as follows:

"when": "jupyterNotebookCellEditorSelected || jupyterCellListCodeSelected"

[DonJayamanne]

Not sure how we can do that. As you toggle between notebooks, we'll need to update our context keys and that's async. i.e the context keys will change only after a few seconds, hence the commands/icons will unnecessarily appear & then disappear or viceversa. I dont think we can create wrappers around vscode context keys

[rchiodo]

I believe Joyce was referring to making context keys that are combinations of others so our when clauses are simpler.

[IanMatthewHuff]

Chatted about this at standup and currently not an easy way to do this, and no current issues with this, so closing.