modules/version: init with nixpkgsReleaseCheck

Inspired by `home.version` and `home.enableNixpkgsReleaseCheck` in Home
Manager. Print a warning when `lib` or `pkgs` are from a different
release to Nixvim.

tests/nixpkgs-module.nix

@@ -24,6 +24,7 @@ let
             runNvim = false;
             runCommand = runCommandLocal;
           };
+          version.enableNixpkgsReleaseCheck = false;
         }
         ../modules/misc
         ../modules/top-level/test.nix

tests/test-sources/modules/version.nix

@@ -0,0 +1,44 @@
+{ pkgs }:
+{
+  invalid-pkgs =
+    { lib, config, ... }:
+    let
+      versionInfo = lib.importTOML ../../../version-info.toml;
+      nixvimRelease = versionInfo.release;
+      pkgsRelease = "<invalid>";
+    in
+    {
+      # The test-suite uses `pkgs = mkForce`, so override it.
+      # Overlay `pkgs` with an invalid `release`:
+      _module.args.pkgs = lib.mkOverride 0 (
+        pkgs.extend (
+          final: prev: {
+            lib = prev.lib.extend (
+              final: prev: {
+                trivial = prev.trivial // {
+                  release = pkgsRelease;
+                };
+              }
+            );
+          }
+        )
+      );
+
+      test.warnings = expect: [
+        (expect "count" 1)
+        (expect "any" "You are using:")
+        (expect "any" "- Nixvim version: ${nixvimRelease}")
+        (expect "any" "- Nixpkgs version used to evaluate Nixvim: ${nixvimRelease}")
+        (expect "any" "- Nixpkgs version used for packages (`pkgs`): ${pkgsRelease}")
+        (expect "any" "If you insist, you can disable this warning using:")
+        (expect "any" "  version.enableNixpkgsReleaseCheck = false;")
+      ];
+
+      assertions = [
+        {
+          assertion = config.version.release == nixvimRelease;
+          message = "Expected `config.version.release` to be ${nixvimRelease}, found ${config.version.release}";
+        }
+      ];
+    };
+}