Add 'new tab' menu item to window system menu when tab strip is hidden

### Description of the new feature

When the tab strip is hidden, there is no clear way to open a new tab. I think that adding a 'new tab' menu item to the window's system menu, which shows the different profiles in a submenu, would fix this.

### Proposed technical implementation details

I don't know much cpp! All I have to say is `AppendMenu()`.

Start by locating the Windows Terminal code that builds the window system menu and uses AppendMenu(). Compare it with the existing new-tab and profile-submenu behavior. Done means the system menu shows a new-tab item with profiles in a submenu when the tab strip is hidden, with appropriate behavior verified in the application.

Written by the indexing model from the issue text.