src/vs/workbench/contrib/terminal/browser/terminalGroup.ts/TerminalGroup/addInstance
class TerminalGroup:	addInstance(shellLaunchConfigOrInstance: IShellLaunchConfig | ITerminalInstance): void {
		let instance: ITerminalInstance;
		if ('instanceId' in shellLaunchConfigOrInstance) {
			instance = shellLaunchConfigOrInstance;
		} else {
			instance = this._terminalService.createInstance(shellLaunchConfigOrInstance);
		}
		if (this._terminalInstances.length === 0) {
			this._terminalInstances.push(instance);
		} else {
			this._terminalInstances.splice(this._activeInstanceIndex + 1, 0, instance);
		}
		this._initInstanceListeners(instance);

		if (this._splitPaneContainer) {
			this._splitPaneContainer!.split(instance, this._activeInstanceIndex + 1);
		}

		instance.setVisible(this._isVisible);

		this._onInstancesChanged.fire();
	}