build(hooks): add commitlint checks for trailers

This change adds checks for the `Change-Id` and `Signed-off-by`
trailers. These are already required by Gerrit, so they have been
configured as warnings as an early heads-up after committing without
needing to push.

This also renames the commitlint configuration file to align it with
the style used by Standard Version when it's eventually introduced:

    `commitlint.config.js` -> `.commitlintrc.js`

Change-Id: I4e3c158e6b3a5407fabd873360d5efce86ebd19e
Signed-off-by: Chris Kay <chris.kay@arm.com>

.commitlintrc.js

@@ -0,0 +1,31 @@
+/*
+ * Copyright (c) 2021, Arm Limited. All rights reserved.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+/* eslint-env es6 */
+
+"use strict";
+
+const cz = require("./.cz.json");
+const { "trailer-exists": trailerExists } = require("@commitlint/rules").default;
+
+module.exports = {
+    extends: ["@commitlint/config-conventional"],
+    plugins: [
+        {
+            rules: {
+                "signed-off-by-exists": trailerExists,
+                "change-id-exists": trailerExists,
+            },
+        },
+    ],
+    rules: {
+        "body-max-line-length": [1, "always", cz.maxLineWidth], /* Warning */
+        "header-max-length": [1, "always", cz.maxHeaderWidth], /* Warning */
+
+        "change-id-exists": [1, "always", "Change-Id:"], /* Warning */
+        "signed-off-by-exists": [1, "always", "Signed-off-by:"], /* Warning */
+    },
+};

commitlint.config.js

@@ -1,14 +0,0 @@
-/* eslint-env node */
-
-"use strict";
-
-const config = require("./.cz.json");
-
-module.exports = {
-    extends: ["@commitlint/config-conventional"],
-    rules: {
-        "header-max-length": [1, "always", config.maxHeaderWidth], /* Warning */
-        "body-max-line-length": [1, "always", config.maxLineWidth], /* Warning */
-        "signed-off-by": [0, "always", "Signed-off-by:"] /* Disabled - buggy */
-    }
-};