feat: add rule for BREAKING CHANGE: in subject

`BREAKING CHANGE:` is a footer thing

Fixed conventional-changelog#3810

Author: abitrolly

@commitlint/rules/src/index.ts

@@ -21,6 +21,7 @@ import {scopeEnum} from './scope-enum';
 import {scopeMaxLength} from './scope-max-length';
 import {scopeMinLength} from './scope-min-length';
 import {signedOffBy} from './signed-off-by';
+import {subjectBreaking} from './subject-breaking';
 import {subjectCase} from './subject-case';
 import {subjectEmpty} from './subject-empty';
 import {subjectFullStop} from './subject-full-stop';
@@ -58,6 +59,7 @@ export default {
 	'scope-max-length': scopeMaxLength,
 	'scope-min-length': scopeMinLength,
 	'signed-off-by': signedOffBy,
+	'subject-breaking': subjectBreaking,
 	'subject-case': subjectCase,
 	'subject-empty': subjectEmpty,
 	'subject-full-stop': subjectFullStop,

@commitlint/rules/src/subject-breaking.test.ts

@@ -0,0 +1,24 @@
+import parse from '@commitlint/parse';
+import {subjectBreaking} from './subject-breaking';
+
+const messages = {
+	empty: 'test: \nbody',
+	filled: 'BREAKING CHANGE: this one',
+};
+
+const parsed = {
+	empty: parse(messages.empty),
+	filled: parse(messages.filled),
+};
+
+test('without subject should succeed', async () => {
+	const [actual] = subjectBreaking(await parsed.empty);
+	const expected = true;
+	expect(actual).toEqual(expected);
+});
+
+test('subject fail with BREAKING CHANGE:', async () => {
+	const [actual] = subjectBreaking(await parsed.filled);
+	const expected = false;
+	expect(actual).toEqual(expected);
+});

@commitlint/rules/src/subject-breaking.ts

@@ -0,0 +1,7 @@
+import {SyncRule} from '@commitlint/types';
+
+export const subjectBreaking: SyncRule = (parsed) => {
+	const result = parsed.subject?.startsWith('BREAKING CHANGE:');
+
+	return [!result, 'move BREAKING CHANGE: to footer'];
+};