i3: add passthru.providedSessions

[lovesegfault]

Motivation for this change

Noticed it was missing following #53843

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

Notify maintainers

cc @modulistic @fpletz @globin

[worldofpeace]

@lovesegfault The i3 module uses services.xserver.windowManager.session, and this is needed only when using sessionPackages.

I did compose a response of how I think this should/can be done, but because you need to hardcode Exec and TryExec in the session files and support cfg.package, it's easier to just generate these.

Though I did notice some things that are not so great in the i3 module

1. deprecate services.xserver.windowManager.i3.extraSessionCommands

(mkRenamedOptionModule
  [ "services" "xserver" "windowManager" "i3" "extraSessionCommands" ]
  [ "services" "xserver" "displayManager" "sessionCommands" ])

2. deprecate services.xserver.windowManager.i3.extraPackages

This is literally an option to use environment.systemPackages, it would be better if there was an option to enable/disable these extraPackages that are like customization for i3 in NixOS.