flake: simplify legacyPackages definitions

- Remove the `makeNixvimWithModule` module arg.
- Move standalone wrapper definitions to `wrappers.nix`
- Move `nixvimConfiguration` alias to `nixvim-configurations.nix`

flake/nixvim-configurations.nix

@@ -1,10 +1,12 @@
 { self, ... }:
 {
   perSystem =
-    { system, ... }:
+    { config, system, ... }:
     {
       nixvimConfigurations.default = self.lib.evalNixvim {
         inherit system;
       };
+
+      legacyPackages.nixvimConfiguration = config.nixvimConfigurations.default;
     };
 }