feat(emacs-pkgs/niri): functions for seamless niri/emacs switching

Introduces a new buffer switching function which is also capable of switching to
existing Emacs frames that already display the target buffer, or to other
windows displayed in the same Niri session.

Not all behaviour is done yet, and there's an explanatory comment in the package
with more details.

Change-Id: I5a548931a681ba32fdb352ecec66845a75268c19
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12535
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tools/emacs-pkgs/niri/default.nix

@@ -0,0 +1,7 @@
+{ depot, ... }:
+
+depot.tools.emacs-pkgs.buildEmacsPackage rec {
+  pname = "niri";
+  version = "1.0";
+  src = ./niri.el;
+}