`show-config` command to inspect resolved configuration

Add a `karva show-config` command that displays the fully resolved configuration, including profile inheritance and per-test overrides:

```
karva show-config
karva show-config --profile ci
karva show-config test-groups
```

This helps debug configuration issues, especially when multiple config sources and profiles are in play.

See: https://nexte.st/docs/running/#other-nextest-commands