fix(commonjs): add .cjs to default file extensions.

packages/commonjs/src/index.js

@@ -34,7 +34,7 @@ import {
 } from './transform';
 
 export default function commonjs(options = {}) {
-  const extensions = options.extensions || ['.js'];
+  const extensions = options.extensions || ['.js', '.cjs'];
   const filter = createFilter(options.include, options.exclude);
   const {
     ignoreGlobal,

packages/commonjs/test/fixtures/samples/cjs-extension/export.cjs

@@ -0,0 +1,3 @@
+module.exports = {
+  test: 42
+};

packages/commonjs/test/fixtures/samples/cjs-extension/main.js

@@ -0,0 +1,3 @@
+const { test } = require('./export.cjs');
+
+console.log(test);

packages/commonjs/test/snapshots/test.js.md

@@ -21,3 +21,24 @@ Generated by [AVA](https://avajs.dev).
     ␊
     module.exports = main;␊
     `
+
+## imports .cjs file extension by default
+
+> Snapshot 1
+
+    `'use strict';␊
+    ␊
+    var _export = {␊
+      test: 42␊
+    };␊
+    ␊
+    const { test } = _export;␊
+    ␊
+    console.log(test);␊
+    ␊
+    var main = {␊
+    ␊
+    };␊
+    ␊
+    module.exports = main;␊
+    `

packages/commonjs/test/test.js

@@ -639,3 +639,12 @@ test('logs a warning when the deprecated namedExports option is used', async (t)
     'The namedExports option from "@rollup/plugin-commonjs" is deprecated. Named exports are now handled automatically.'
   );
 });
+
+test('imports .cjs file extension by default', async (t) => {
+  const bundle = await rollup({
+    input: 'fixtures/samples/cjs-extension/main.js',
+    plugins: [commonjs()]
+  });
+  const code = await getCodeFromBundle(bundle);
+  t.snapshot(code);
+});