improve gitignore, make niri shared config more modular

dotfiles/shared/niri/biri-shared.kdl

@@ -0,0 +1,16 @@
+binds {
+    Mod+Slash { spawn "niri-scratch-workspace-toggle"; }
+    Mod+Shift+Slash { move-window-to-workspace "x" focus=false; }
+}
+workspace "x" {
+    hidden true
+    layout {
+        focus-ring {
+            width 3
+            active-color "#a33d3d"
+        }
+    }
+}
+
+spawn-at-startup "niri-listen-for-scratch-workspace-change"
+spawn-at-startup "niri-clear-state"