shells: make barebones rust shell and create additional screeps shell

shells/rust-screeps/default.nix

@@ -0,0 +1,35 @@
+{
+  # Snowfall Lib provides a customized `lib` instance with access to your flake's library
+  # as well as the libraries available from your flake's inputs.
+  lib,
+  # You also have access to your flake's inputs.
+  inputs,
+
+  # The namespace used for your flake, defaulting to "internal" if not set.
+  namespace,
+
+  # All other arguments come from NixPkgs. You can use `pkgs` to pull shells or helpers
+  # programmatically or you may add the named attributes as arguments here.
+  pkgs,
+  mkShell,
+  system,
+  ...
+}:
+let
+  fenix =
+    with inputs.fenix.packages.${system};
+    combine [
+      latest.toolchain
+      targets.wasm32-unknown-unknown.latest.rust-std
+    ];
+in
+mkShell {
+  # Create your shell
+  nativeBuildInputs = [
+    fenix
+    pkgs.llvmPackages.bintools
+    pkgs.wasm-pack
+  ];
+
+  CARGO_TARGET_WASM32_UNKNOWN_UNKNOWN_LINKER = "lld";
+}