feat: add nix flake

[zakuciael]

Description

This PR adds a new installation method for NixOS users by adding a flake.nix file.
It also adds a new GitHub action that automatically checks if the nix configuration is valid, and that the packages can be built.
The Flake requires no additional updates from maintainers as it gathers all themes located under the assets/backgrounds/ folder and builds them automatically.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• I've updated the documentation to reflect the changes.

[AdisonCavani]

Hello!
Thank you for your contribution.
Please move the files into build folder (nix related).

[zakuciael]

I've moved the files to the build/ directory. But I've left the flake.nix and flake.lock in the root. Otherwise, it adds extra steps when installing the flake on NixOS and it is a standard in Nix community to add it to the root of the repository.