chore(tooling): add nix flake and reconfigure some stuff for neovim

flake.nix

@@ -0,0 +1,41 @@
+{
+  description = "A Nix-flake-based C# development environment";
+
+  inputs.nixpkgs.url = "https://flakehub.com/f/NixOS/nixpkgs/0.1.*.tar.gz";
+
+  outputs =
+    { self, nixpkgs }:
+    let
+      supportedSystems = [
+        "x86_64-linux"
+        "aarch64-linux"
+        "x86_64-darwin"
+        "aarch64-darwin"
+      ];
+      forEachSupportedSystem =
+        f:
+        nixpkgs.lib.genAttrs supportedSystems (
+          system:
+          f {
+            pkgs = import nixpkgs { inherit system; };
+          }
+        );
+    in
+    {
+      devShells = forEachSupportedSystem (
+        { pkgs }:
+        {
+          default = pkgs.mkShell {
+            packages = with pkgs; [
+              dotnet-sdk_9
+              omnisharp-roslyn
+              mono
+            ];
+            shellHook = ''
+              export ASPNETCORE_ENVIRONMENT=Development
+            '';
+          };
+        }
+      );
+    };
+}