Add `rbld -t` for checking if the code syntax is valid

[llakala]

Done via nix eval /etc/nixos#nixosConfigurations.framework.config.system.build.toplevel. Just checks that the code is syntactically correct, doesn't activate it or anything.

Unfortunately, for this to work, we'd need access to the current host. I could do this manually like I handle $CONFIG_DIRECTORY, but honestly, I should really just move the rbld script to within a nix file.

[llakala]

rbld script has been moved into a nix file as of 2a8f2da4119a22bb475115f548604c2f4007150a. And, syntax highlighting works, but only within Helix (this is good, makes me stop using VSCode). We can now add this functionality easily, and anything else.

[llakala]

Yet another abstraction: the rebuild script has been moved to its own repo. However, I'm facing lots of theoretical questions now, like:

• Should I provide an argument for setting the config directory?
• How should I provide an argument for setting the config directory?
• What if the user uses unstable by default, and stable as a secondary package set? Do I need an option for rbld -f?
• Does the rbld -f functionality even make sense within the rbld function?
• Why am I worrying about all these things for a project with zero stars?

This issue may languish a bit until I feel more comfortable with the state of the script.

[llakala]

Progress:

• We can now pass a config directory via -d. It's set to /etc/nixos if no path is passed. Ideally, we'd add a custom error here to check that the directory exists, has a flake, and a git repo, rather than getting an inscrutable error.
• We're planning on splitting the flake commands off into a separate function, unify. unify stands for: "Update NixOS Inputs For Yourself".
• Positional parameters bad. Once above is done, the commands should ideally only take arguments. We can then work on adding more arguments, for things like specifying the flake inputs we care about.
• ADD DOCUMENTATION. it's not that hard.

Once all that is done, we can work on new functionality, and implement stuff like rbld -t.