Add Nix shell configuration for compiling using Nix

[r6915ee]

This pull request simply adds a Nix development shell configuration in a shell.nix file to allow compiling and testing using the Nix package manager. This is especially useful on NixOS, which is hosted alongside Nix.

The added file tells Nix to do the following operations when opening a development shell:

• Download an older tarball of the official Nixpkgs channel for version locking, specifically with Haxe in mind. This tarball is theoretically the first one to contain Haxe 4.2.5.
• Install a list of libraries:
  • SDL2
  • pkg-config
  • openal
  • alsa-lib (Both OpenAL and alsa-lib are installed in order to prevent the system from using OSS for audio, which is a no-go on NixOS because it doesn't contain the typical packages that are used for emulating OSS...and also because OSS is practically deprecated nowadays.)
  • libvlc (Of course!)
  • libpulseaudio (Lime currently only uses PulseAudio, so the program will hang without this if it's excluded, ESPECIALLY because it's not linked.)
  • libGL
  • Various natural X11 libraries. These can be seen by checking the second half of the libs variable.
• Installs Haxe and Neko. For good measure, this also installs libsForQt5.qttools, which is the package for qttools available in this tarball.
• Configure LD_LIBRARY_PATH to use the proper library paths, since Lime apparently checks this environment variable for additional libraries explicitly. This also includes SDL2.
• sources the update script in the shell hook, which is run on startup of the development shell.

Do note that, although Nix supports macOS natively, I am unable to test it because I do not have a macOS device of any kind of my own (although the usage of certain libraries and how they'll work on macOS, or any Darwin variant for that matter, is questionable).

Additionally, the Nix shell configuration is intended for me to compile for another pull request, which I will create in due time. However, that branch will have the main branch of my contribution fork as the ancestor, so no commits from this branch will be used.