Add context key for current workspace folder

[isc-bsaviano]

I would like to add a when clause context to some of my extension's menu options that depends on properties of the workspace folder. I planned to use in operator and have my extension set a context key with an array of all workspace folder URIs that satisfy my condition. However, there is no context key for the current workspace folder. For reference, these options are in the commandPalette, editor/title, editor/context, and explorer/context menus.