feat: add Nix flake package, AppImage and .deb packaging

- flake: build teleportfling and teleportfling-gui as nixpkgs packages
  (buildGoModule) with a .desktop entry and icon for the GUI
- packaging/appimage.sh: bundle the GUI + runtime libs (recursive ldd)
  into a self-contained AppImage via appimagetool
- packaging/deb.sh: build a .deb for Debian/Ubuntu with both binaries,
  desktop entry, icon and declared runtime dependencies
- README: document all three packaging methods
- .gitignore: ignore dist/

packaging/AppRun

@@ -0,0 +1,5 @@
+#!/bin/sh
+# AppImage AppRun: locate bundled libraries and launch the GUI.
+HERE="$(dirname "$(readlink -f "$0")")"
+export LD_LIBRARY_PATH="${HERE}/usr/lib${LD_LIBRARY_PATH:+:$LD_LIBRARY_PATH}"
+exec "${HERE}/usr/bin/teleportfling-gui" "$@"