Extract overrides option to Home Manager

[brckd]

As described in #25, importing nix-flatpak in other third-party modules that import home-manager creates problems. This also applies for Stylix with its pending support for theming Flatpak apps as hinted in danth/stylix#465 (comment). Therefore Stylix will currently use an incompatible in-house solution as described in danth/stylix#693 (comment).

One solution to this problem is to extract the user override options into Home Manger. This would eliminate the need to import nix-flatpak into Stylix in the first place and makes modules using the option compatible. The code cannot be extracted directly, because this project's and Home Manager's license and copyright are not compatible. A new solution would need to be written for Home Manager, but this would fragment the code for home and NixOS overrides.

Please let me know whether this is a desirable solution that can be adapted by this project.