Add API support for submenus

[egamma]

Add support for submenus to simplify the menu structure of the Git extensions '...' menu.

[gjsjohnmurray]

See #9827

I'd particularly like #9827 (comment) to be catered for in the new API, so an extension can contribute a submenu whose contents are computed at display-time.

[joaomoreno]

Syntax proposal:

{
  "contributes": {
    "menus": {
      "scm/title": [
        {
          "command": "git.commit",
          "group": "navigation"
        },
        {
          "submenu": "git.stage",
          "group": "navigation"
        }
      ],
      "git.stage": [
        {
          "command": "git.stageAll",
          "group": "navigation"
        },
        {
          "command": "git.stageUntracked",
          "group": "navigation"
        }
      ]
    },
    "submenus": [
      {
        "id": "git.stage",
        "label": "Stage",
        "icon": "$(something)"
      }
    ]
  }
}

[gjsjohnmurray]

Syntax proposal:

@joaomoreno Maybe extend the syntax of submenus contribution to support submenus that call the extension at display-time to obtain their contents.

[joaomoreno]

That will definitely not be in there for the first iteration. That should even be generalized for regular menus.