feat(tazjin/dotfiles): check in Niri configuration

Adds the current (fairly simple) Niri configuration, which is annealed from
a few days of being poked constantly.

This isn't yet deployed through home-manager or anything as I'm still doing
stuff with it manually.

Change-Id: I3dfcafb55d98d9655585d3128e1088fc13e60cc7
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12434
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
Autosubmit: tazjin <tazjin@tvl.su>

users/tazjin/dotfiles/default.nix

@@ -1,3 +1,4 @@
 _: {
   dunstrc = ./dunstrc;
+  niri = ./niri.config.kdl;
 }