Support terminals to be launched from extensions but not show in panel

[Tyriar]

Allow terminals to be launched in backgrounded/invisible, until the user decides to show it.

interface TerminalOptions {
	/**
	 * When enabled the terminal will run the process as normal but not be surfaced
	 * to the user until `Terminal.show` is called. The typical usage for this
	 * is when you need to run something that may need interactivity but only
	 * want to tell the user about it when interaction is needed.
	 */
	runInBackground?: boolean;
}