add kak-session script

pkgs/kak-session/default.nix

@@ -0,0 +1,44 @@
+{ ... }:
+let
+  sources = import ../../nix/sources.nix;
+  nixpkgs = import sources.nixpkgs { };
+in with nixpkgs;
+stdenv.mkDerivation {
+  name = "kak-session";
+
+  src = ./.;
+
+  buildPhase = ''
+    cat > kak-session.sh <<EOF
+    #!/usr/bin/env bash
+    set -euo pipefail
+
+    # find the git root directory for the project
+    ROOT=\$(pwd)
+    while ! test -d "\$ROOT/.git" && test "\$ROOT" != "/"; do
+      ROOT=\$(dirname \$ROOT)
+    done
+
+    if test "\$ROOT" = "/"; then
+      ROOT=\$(pwd)
+    fi
+
+    SESSION=\$(basename \$ROOT | sed 's/\./-/g')
+
+    if ! ${pkgs.kakoune}/bin/kak -l | grep -q "\$SESSION"; then
+      ${pkgs.kakoune}/bin/kak -d -s "\$SESSION"
+    fi
+
+    exec ${pkgs.kakoune}/bin/kak -c "\$SESSION" \$@
+    EOF
+  '';
+
+  installPhase = ''
+    mkdir -p $out/bin
+    cp kak-session.sh $out/bin/kak-session
+    chmod +x $out/bin/kak-session
+
+    ln -s $out/bin/kak-session $out/bin/kak
+    chmod +x $out/bin/kak
+  '';
+}