flake: add devShells

exposed via nix registry :D
can be used in nix-direnv like `use flake shells#cs.osu`

shells/cs/default.nix

@@ -0,0 +1,14 @@
+{
+  callPackage,
+  lib,
+  mkShell,
+  dotnetCorePackages
+}:
+mkShell {
+  nativeBuildInputs = [ (with dotnetCorePackages; combinePackages [ sdk_6_0 runtime_6_0 ]) ];
+  DOTNET_CLI_TELEMETRY_OPTOUT = 1;
+
+  passthru = {
+    osu = callPackage ./osu.nix {};
+  };
+}