Add support for '__layout.tsx' in the router-cli

[jjones315]

Currently in the router-cli the layout lives outside of the folder it applies to. For example in the 'kitchen-sink-route-gen' example folder structure looks something like

├── src
│   ├── routes
│   │   ├── authenticated
│   │   │   ├── ...ect
│   │   ├── dashboard
│   │   │   ├── index.tsx
│   │   │   ├── invoices.tsx
│   │   │   ├── ...ect
│   │   ├── authenticated.tsx
│   │   ├── dashboard.tsx

as the application grows and more of these "feature" folders are created it would be nice to avoid jumping in the out of the folder. To me the ideal structure would look something like this.

├── src
│   ├── routes
│   │   ├── dashboard
│   │   │   ├── __layout.tsx
│   │   │   ├── index.tsx
│   │   │   ├── invoices.tsx
│   │   │   ├── ...ect
│   │   ├── authenticated
│   │   │   ├── __layout.tsx
│   │   │   ├── ...ect

I can create a PR with this feature but wanted to open discussion around making the __layout.tsx file name essentially reserved.