refactor(dev-infra): refactor commit-message files (#38845)

Refactor the commit-message files to be consistent with how other ng-dev tooling
is structured.

PR Close #38845

dev-infra/commit-message/commit-message-source.ts

@@ -0,0 +1,13 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+/**
+ * The source triggering the git commit message creation.
+ * As described in: https://git-scm.com/docs/githooks#_prepare_commit_msg
+ */
+export type CommitMsgSource = 'message'|'template'|'merge'|'squash'|'commit';