Add cjs as a default extension (#11498)

packages/babel-core/src/index.js

@@ -48,6 +48,7 @@ export const DEFAULT_EXTENSIONS = Object.freeze([
   ".es6",
   ".es",
   ".mjs",
+  ".cjs",
 ]);
 
 // For easier backward-compatibility, provide an API like the one we exposed in Babel 6.

packages/babel-register/test/index.js

@@ -41,7 +41,7 @@ jest.mock("source-map-support", () => {
 });
 
 const defaultOptions = {
-  exts: [".js", ".jsx", ".es6", ".es", ".mjs"],
+  exts: [".js", ".jsx", ".es6", ".es", ".mjs", ".cjs"],
   ignoreNodeModules: false,
 };