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/

.gitignore

@@ -6,3 +6,4 @@
 # nix
 result
 result-*
+dist/