Support niri validate

[jgarte]

Hi,

This is a stub to support the niri validate command.

References and Mentions to niri validate

https://yalter.github.io/niri/Configuration%3A-Introduction.html#loading

https://wiki.archlinux.org/title/Niri#Configuration

https://github.com/YaLTeR/niri/wiki/Configuration:-Introduction#loading