Test menu contributions

[jrieken]

Tests #3192

• any os - @weinand for OS X
• any os - @kieferrm

We added a new contribution point called menus which allows extension writers to contribute to different (context) menus. Make sure that

• you get proper intellisense and documentation when authoring package.json
• you get proper validation when authoring package.json
• you get warnings/errors when launching with a bogus menus-contributions
• you get warnings when a menu item references a missing command
• you can contribute an icon to commands in the commands section
• an icon can be a path or a themable icon which is two values for the light or dark theme
• you can contribute menu items to editor/title, editor/context, and explorer/context
• a menu item can define a when condition under which it shows
• the when is the same as the keybindings-when and can use the same keys
• the when can use two new keys resourceScheme and resourceLangId which are available without an editor - think of the explorer context menu
• the when can be a boolean configuration value, e.g config.editor.lineNumbers
• menus update when keys change, e.g. editorHasSelection
• a menu item can declare an alternative command which shows when pressing alt and hovering (not in context menus)
• a menu item can define a group which is used to group and separate the item from others
• there is a special group called navigation which means that the menu item is moved to the top of the context menus or in the title and not overflow area of editor actions
• the group can be like navigation@10 where the @10 defines the sorting inside that group
• the command is passed the URI of the selected resource - the file from the explorer or the document uri from the editor

[thekalinga]

@jrieken Is there a way currently to contribute commands to exclusively to explorer context menu & add a shortcut for it.

For eg., if I select a folder in explorer & press alt+insert, I want to add the ability for the user to select a set of options the way intellij show.