ref(nix): move shell to shell.nix

shell.nix

@@ -0,0 +1,24 @@
+{
+  self',
+  packageToml,
+  rust-bin,
+  mkShell,
+}: let
+  msrv = packageToml.rust-version;
+
+  mkDevShell = rust:
+    mkShell {
+      inputsFrom = [
+        (self'.packages.default.override {
+          rust = rust.override {
+            extensions = ["rust-src"];
+          };
+        })
+      ];
+
+      env.RUST_BACKTRACE = 1;
+    };
+in {
+  default = mkDevShell rust-bin.stable.latest.default;
+  msrv = mkDevShell rust-bin.stable.${msrv}.default;
+}