Add command for opening InterSystems documents

[isc-bsaviano]

This PR fixes #1379. The new Open InterSystems Document... command can be opened from the command palette and it provides a UI for browsing all files in the connected server-namespace. The user can either pick an item in the list, or type text into the filter box and press "Enter" to submit that. The UI should function very similarity to the built-in simple file dialog. I would like to assign a keybinding to this command, but I couldn't find a good one so I'm open to suggestions.