Add Nix flake

[myclevorname]

Here are some differences in the flake people might need to know.

1. The comment in the resulting programs is "CE C Toolchain" by default and not the current time to ensure reproducibility.
2. fasmg, conv, and ez80- are not included in the bin/ folder, but I overrode the paths to said programs in src/makefile.mk to be absolute paths to the actual programs within the Nix store. If you want access to those, use a devshell, like the one in examples/nix-flake, which stores a template for creating a new program with Nix.
3. The cedev-nightly package has been tested, but the cedev-stable package hasn't been tested.
4. When writing CE C programs with Nix, you can manually toggle to the stable or nightly toolchains using the stableToolchain flag.
5. Instead of typing nix build .#cedev-nightly, you have to type .?submodules=1#cedev-nightly because Nix doesn't fetch submodules by default for some odd reason.
6. The Flake file looks like a mess and may need to be cleaned up.

[mateoconlechuga]

There are so many things wrong here.