Consider removing `pika::init`

[msimberg]

Removing pika::init could simplify starting the runtime noticeably and make spawning of work on the runtime more explicit (usually a good thing). It would remove the special casing of pika_main as the "entry point" to an application, including the pika_main = nullptr special case. It would leave us with the more symmetric pika::start and pika::stop which I find more intuitive to reason about.

On the other hand it slightly complicates command line handling for users since they may have to deal with pika command line options. Users also have to be more careful (explicit) about spawning work onto the pika runtime/a thread pool.