support VS Code when closure contexts

[akosyakov]

https://code.visualstudio.com/docs/getstarted/keybindings#_when-clause-contexts

[akosyakov]

For instance, issues from deploying debug-node VS Code extension:

root ERROR Unrecognized key 'F10' in 'F10'
root ERROR Cannot read property 'split' of undefined
root WARN Could not register keybinding:
  {"command":"extension.node-debug.startWithStopOnEntry","keybinding":"F10","context":"!inDebugMode && debugConfigurationType=='node'"}
Error: Unrecognized key 'F10' in 'F10'
root WARN Could not register keybinding:
  {"command":"extension.node-debug.startWithStopOnEntry","context":"!inDebugMode && debugConfigurationType=='node'"}
TypeError: Cannot read property 'split' of undefined

[akosyakov]

Default contexts:

• Editor contexts - provided by Monaco
• Operating system contexts
• Mode contexts
• Resource contexts
• Explorer contexts
• Editor widget contexts - provided by Monaco
• Integrated terminal contexts
• Global UI contexts
• Configuration settings contexts -> can be provided by Monaco: replace default Monaco configuration service with custom based on PreferenceService #4073
• Active/focused view or panel contexts -> support active/focused view or panel when closures #4104