Flags / VSCode for outputting phase results/inputs

In continuation of #303, I also often find myself wanting to know the intermediate results between the phases (core pre/post Optimizer, PolymorphismBoxing etc).
It would be really nice for debugging if this was possible without needing to change the compiler.

Example usage:
```
$ effekt --show-IR-after Optimizer ./test.effekt
... outputs IR
$ effekt --show-IR-before Optimizer ./test.effekt
... outputs IR
```

Maybe we can somehow achieve this with the existing phase abstraction?