Expose integrated terminal extension API

[Tyriar]

The initial extension API will allow:

• Creating and optionally naming of a terminal instance
• Sending commands (text sequences + '\n') to these "owned" terminal instances

Additional APIs will be considered via feedback from consumers.

Related: #547

[jrieken]

please keep me in the loop

[Tyriar]

The current plan is to be able to create terminals and send text to them:

CreateTerminal(name?: string): Terminal
Terminal.runCommand(text: string): void

@weinand what sort of API would you expect for the debugging support which we eventually want (#10514)?

[jrieken]

I would also suggest dispose, show, hide, name, like so

interface Terminal {
  name:string;
  runCommand(cmd:string)
  show(preserveFocus: boolean): void
  hide(): void;
  dispose(): void;
}

Also, I wonder if runCommand can return a promise that signals when execution is done, resolving to the exit code?

[weinand]

@Tyriar the proposed API seems to cover most of our needs.
Ideally it would be great if Terminal.runCommand() would return the process id of the command, but that's probably too tricky.
And a promise that signals when execution is done would be helpful too.