tools: Add commit linter

Run under Github Actions, and restrict commit messages, structure, with, etc.

Enforce using only our specified prefixes for commit messages. It reduces the
work release managers do when _crafting_ release notes.

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>

.github/commitlint.config.js

@@ -0,0 +1,44 @@
+module.exports = {
+	rules: {
+		'header-max-length': [2, 'always', 72],
+		'type-case': [2, 'always', 'lower-case'],
+		'type-empty': [2, 'never'],
+		'type-enum': [
+			2,
+			'always',
+			[
+				'babeld',
+				'bfdd',
+				'bgpd',
+				'doc',
+				'docker',
+				'eigrpd',
+				'fpm',
+				'isisd',
+				'ldpd',
+				'lib',
+				'multi',
+				'nhrpd',
+				'ospf6d',
+				'ospfd',
+				'pbrd',
+				'pimd',
+				'pim6d',
+				'ripd',
+				'ripngd',
+				'sharpd',
+				'staticd',
+				'tests',
+				'tools',
+				'vtysh',
+				'vrrpd',
+				'yang',
+				'zebra',
+				'all',
+			],
+		],
+		'subject-empty': [2, 'never'],
+		'subject-full-stop': [2, 'never', '.'],
+		'subject-case': [2, 'always', 'sentence-case'],
+	},
+};

.github/workflows/commitlint.yml

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+name: commitlint
+
+on:
+  pull_request_target:
+    types:
+      - opened
+      - reopened
+
+jobs:
+  lint:
+    if: github.repository == 'frrouting/frr'
+    permissions:
+      contents: read
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check Commit
+        uses: wagoid/commitlint-github-action@v5
+        with:
+          configFile: .github/commitlint.config.js