feat(nix-based): Bext Check

[tulilirockz]

A command for checking if a provided system extension is valid, and explaining why it isn't if not.

The user would do something like

bext layer check [FILE]

And the command would open up the layer and check out what is right or wrong, like LAYER/usr/lib/extensions.d/LAYER.sysext has the same name as the target layer, or if there is a LAYER/etc directory, for example.