Add a top-level `Terminal` menu for better discoverability

[Tyriar]

The purpose of a new "Terminal" menu would be to boost discoverability of the terminal itself and some of its features.

[Tyriar]

WIP pushed to https://github.com/Microsoft/vscode/tree/tyriar/49477_terminal_menu

[Tyriar]

I more cut down alternative:

[Tyriar]

As part of this, some screenshots should be updated on the website to include the new menu.

[chpxu]

I personally feel that just adding a 'Terminal'button in the status bar at the bottom could. do some good too, since whilst I do like this top-level menu (first coming to VSCode, I had no idea how to open the terminal), a simple button which auto shows the terminal (or equivalent to dragging from bottom and clicking terminal/Create terminal [? ]) could be just as effective - this is a personal opinion however

[nijikokun]

I agree with @beastdestroyer because when you're using applications in full screen and using the four finger swipe mode with macOS, this menu is not visible unless you move your curser to the top, and for the majority of applications in this working environment you'll never do that except by accident.

Having it closer to the terminal and on the status bar would be much better.