add programs.niri.extraConfig

[cmm]

I found myself wanting to enable the recent-windows feature in the unstable niri, but it's not covered by the settings module yet and all my existing niri configuration uses the module. So I thought that having the traditional extraConfig option that allows one to combine proper modular configuration with verbatim sections would be cool, so here goes.

[aberter0x3f]

Cool feature! When will it be merged into the main branch?

[cmm]

the need for something like this is arguably less pressing now that you can just include things (EDIT I mean you could include things if there were a module option for it, which there isn't presently either, but that's a much easier sell)

[xinyifly]

I found this pull request useful for me because I need include dank-material-shell's generated kdl configs. Especially I need include optional=true from niri unstable to satisfy nix build from niri-flake's config check where the dank-material-shell's kdl configs doesn't exist either during nix build or before dank-material-shell's first start.