Add option to only generate the aoc-autobuild source

[mjpieters]

To make it easier to debug with the runner, please add an option to only generate the runner binary code (so let us skip execution of cargo run). Something like cargo aoc generate or an extra switch for cargo aoc ...

That way I can tell VSCode / LLDB to pre-run cargo aoc generate -d ... before then starting a debug session from the aoc-autobuild binary.