chore(git): populate .gitmessage

Populate the default Git commit message with information about the
Conventional Commits specification.

See https://www.conventionalcommits.org.

home-manager/modules/common.nix

@@ -76,6 +76,23 @@ in
 
   home.packages = (import ./home-manager-packages.nix) { inherit pkgs; };
 
+  home.file.".gitmessage".text = ''
+    type(scope): description
+
+    # type: fix, feat, build, chore, ci, docs, style, refactor, perf, test
+    #
+    # Description
+    #
+    # - Why is this change necessary?
+    # - How does it address the issue?
+    # - What side effects does this change have?
+    #
+    # For breaking changes, add an ! after the type/scope, uncomment the following
+    # line and describe the change:
+    #
+    # BREAKING CHANGE:
+  '';
+
   home.file."nodemon.json".text = ''
     {
       "ignore": [