$psEditor commands should be registered as VS Code commands

[daviwil]

Currently commands that are registered using the $psEditor API are not being registered as commands with VS Code. We'll need to start registering them so that we can take advantage of new extensibility points like context menus: PowerShell/PowerShellEditorServices#290

[gerane]

Would this also add the commands to the main list of extension commands and allow them to be set via keybinding?