feat(emacs): Add telega.el support

Packages the telega-server binary and adds the required mode into
Emacs.

Unread message count is displayed in the modeline, which is neat.

Probably need to figure out some key bindings for this.

third_party/telega/default.nix

@@ -0,0 +1,22 @@
+# Telega is an Emacs client for Telegram. It requires a native server
+# component to run correctly, which is built by this derivation.
+{ pkgs, ... }:
+
+with pkgs.third_party;
+
+stdenv.mkDerivation {
+  name = "telega";
+  buildInputs = [ tdlib ];
+
+  src = fetchFromGitHub {
+    owner = "zevlg";
+    repo = "telega.el";
+    rev = "d532b16067cf24728a2aa03a7aeaebe2ceac7df4";
+    sha256 = "1s2sd07sin9sy833wqprhbfk5j1d1s4azzvj6d8k68sxlgz8996m";
+  } + "/server";
+
+  installPhase = ''
+    mkdir -p $out/bin
+    mv telega-server $out/bin/
+  '';
+}