Use CLI subcommands

We have accumulated a lot of CLI flags, but many of them are just exclusive _commands_.
https://github.com/effekt-lang/effekt/blob/7de9eff01bf7ab87059a564dea4e7a103b7145e7/effekt/jvm/src/main/scala/effekt/EffektConfig.scala#L8-L165

Instead, we might want to move to subcommands, so something like:
```sh
effekt run
effekt compile --backend=
effekt lsp
... # etc
```
This would also allow us to then add other commands like `effekt fmt` for formatting, `effekt watch` for rebuilding files upon file change, and `effekt doc` for documentation generation in the future.