Add 'menus' contribution point for plugins

[azatsarynnyy]

This PR allows a plugin to contribute an item to the menus:

• navigator context menu;
• editor context menu.

Optionally, a menu group and an item order within the group can be specified.

Example of the contribution:

"contributes": {
  "menus": {
    "editor/context": [
      {
        "command": "command.id-1",
        "group": "save@4"
      }
    ],
    "explorer/context": [
      {
        "command": "command.id-2",
        "group": "navigation@2"
      }
    ]
  }
}

[azatsarynnyy]

@benoitf could you please take a look at this PR

[benoitf]

nice