add cxx templates and improve rust template

templates/cxx/flake.nix

@@ -0,0 +1,34 @@
+{
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+    flake-utils = {
+      url = "github:numtide/flake-utils";
+    };
+  };
+  outputs = {
+    self,
+    flake-utils,
+    nixpkgs,
+    ...
+  }:
+    flake-utils.lib.eachDefaultSystem (system: let
+      inherit (pkgs) lib;
+      pkgs = import nixpkgs {
+        inherit system;
+      };
+    in {
+      formatter = pkgs.alejandra;
+      devShells.default = pkgs.mkShell {
+        packages = with pkgs; [
+          clang-tools
+
+          gcc
+          bear
+          gnumake
+        ];
+      };
+    });
+  nixConfig = {
+    keepOutputs = true;
+  };
+}