[Nix] Add overlays to flake

[matt1432]

1. Moved the inputs at the top of the flake like in the ags flake.
2. Added the systems input. You can read on why this is beneficial here: https://github.com/nix-systems/nix-systems
3. Added overlays of all the packages under the astalLibs namespace. This allows cleaner nix code and ensures users will have compatible versions of dependencies in their system. For example, using overlays for astal and ags fixes nix lib.bundle with gtk4 does not work ags#678 for me
4. Changed the forAllSystems function to my own so it makes the function param pkgs to simplify the use of the overlays

I have a PR for AGS as well if this gets merged. Of course the new function name and namespace name can be changed if you wish

[matt1432]

The only thing not making use of the overlays is the lua function but I don't use it so I didn't want to mess it up