[red-knot] Keep subcommands optional for the binary (#12715)
## Summary
This PR updates the `red_knot` CLI to make the subcommand optional.
## Test Plan
Run the following commands:
* `cargo run --bin red_knot --
--current-directory=~/playground/ruff/type_inference` (no subcommand
requirement)
* `cargo run --bin red_knot -- server` (should start the server)
