feat: initialise Nix tooling

Adds a package, and a dev shell for Discorss. `nix run .#` and `nix develop` will be allowed in the current mechanism, but direnv users can simply use `direnv allow` to enter a shell directly.

nix/package.nix

@@ -0,0 +1,53 @@
+{
+  lib,
+  stdenvNoCC,
+  makeWrapper,
+  python312,
+  ...
+}:
+stdenvNoCC.mkDerivation (finalAttrs: {
+  pname = "discorss";
+  version = "0.0.1";
+
+  # Point to the repository root for the source. `builtins.path` is used to construct
+  # a pure and reproducible source path that doesn't change based on the name
+  # of the directory our source code lives in. Alternative to this is to just use it
+  # directly, as follows:
+  # `src = ../.;`
+  src = builtins.path {
+    path = ../.;
+    name = "discorss"; # the directory will always be named discorss in the eyes of Nix
+    filter = lib.cleanSourceFilter; # avoid copying junk files and vsc dirs to the store
+  };
+
+  # programs and libraries *used at build-time* that, if they are a compiler or similar tool, produce
+  # code to run at run-time. In other words, tools used to build the new derivation
+  # Alternative is buildInputs, which includes programs and libraries used by the new derivation at run-time
+  nativeBuildInputs = [makeWrapper];
+
+  # Normally propagatedBuildInputs get, well, propagated to the final build. This is not the
+  # case for s since we omit a handful of shell hooks that would handle it for us. We can
+  # still use propagatedBuildInputs to remain close to the standard, but we will have to
+  # pass it to the wrapper manually.
+  propagatedBuildInputs = [
+    # Create a python wrapper with the dependencies that we need.
+    # This will produce a /nix/store/some-hash-python/bin/python3 that has the modules
+    # our program needs.
+    (python312.withPackages (ps: with ps; [feedparser requests]))
+  ];
+
+  dontUnpack = true; # source is not an archive
+  dontConfigure = true; # there is no configure script
+  dontPatch = true; # we are building from source, patches are redundant
+
+  buildPhase = ''
+    runHook preBuild
+
+    # Create a wrapper for Discorss with the specific version of Python (with dependencies)
+    # added to its PATH.
+    makeWrapper $src/discorss.py $out/bin/discorss \
+      --prefix PATH : "${lib.makeBinPath finalAttrs.propagatedBuildInputs}"
+
+    runHook postBuild
+  '';
+})