Convert `custom-server-express` example to TypeScript (#42110)
Converted Express example to TypeScript to match Contribution docs.
- Added `nodemon` to match the `custom-server` example, and allow live reloading of `server.ts`
- Updated README with explainer for non-standard `<Link href="/a" as="/b">` routing
## Documentation / Examples
- [X] Make sure the linting passes by running `pnpm build && pnpm lint`
- [X] The "examples guidelines" are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)
