feat: add possibility to extend from string (#865)

Author: onrige

@commitlint/resolve-extends/src/index.test.ts

@@ -30,6 +30,17 @@ test('fails for missing extends', async () => {
 	);
 });
 
+test('resolves extends for single config', () => {
+	const input = {extends: 'extender-name'};
+	const ctx = {
+		resolve: id,
+		require: jest.fn(() => ({}))
+	} as ResolveExtendsContext;
+	resolveExtends(input, ctx);
+
+	expect(ctx.require).toHaveBeenCalledWith('extender-name');
+});
+
 test('uses empty prefix by default', () => {
 	const input = {extends: ['extender-name']};
 	const ctx = {

@commitlint/resolve-extends/src/index.ts

@@ -13,7 +13,7 @@ export interface ResolvedConfig {
 
 export interface ResolveExtendsConfig {
 	parserPreset?: unknown;
-	extends?: string[];
+	extends?: string | string[];
 	[key: string]: unknown;
 }
 
@@ -48,7 +48,10 @@ function loadExtends(
 	config: ResolveExtendsConfig = {},
 	context: ResolveExtendsContext = {}
 ): ResolvedConfig[] {
-	return (config.extends || []).reduce<ResolvedConfig[]>((configs, raw) => {
+	const {extends: e} = config;
+	const ext = e ? (Array.isArray(e) ? e : [e]) : [];
+
+	return ext.reduce<ResolvedConfig[]>((configs, raw) => {
 		const load = context.require || require;
 		const resolved = resolveConfig(raw, context);
 		const c = load(resolved);