Add possibility to configure how extensions should be loaded

Fixes avajs#2345 by adding an object form for the `"extensions"` configuration.

This implementation:

1. if `"extensions"` is not defined uses the default module configuration for `mjs`, `cjs`, and `js`
2. generates the module types from the array form of the `extensions` configuration keeping the default module type for `mjs`, `cjs`, and `js` if they're defined and uses `"commonjs"` for everything else, thus maintaining backwords compatibility.
3. generates the module types from the object form of the `extensions` configuration and prevents changing the module type for `mjs`, `cjs`, and `js` extensions (for `js` the `"type"` field cannot be overridden).

lib/cli.js

@@ -267,6 +267,7 @@ exports.run = async () => { // eslint-disable-line complexity
 	const TapReporter = require('./reporters/tap');
 	const Watcher = require('./watcher');
 	const normalizeExtensions = require('./extensions');
+	const normalizeModuleTypes = require('./module-types');
 	const {normalizeGlobs, normalizePattern} = require('./globs');
 	const normalizeNodeArguments = require('./node-arguments');
 	const validateEnvironmentVariables = require('./environment-variables');
@@ -284,12 +285,6 @@ exports.run = async () => { // eslint-disable-line complexity
 
 	const {type: defaultModuleType = 'commonjs'} = pkg || {};
 
-	const moduleTypes = {
-		cjs: 'commonjs',
-		mjs: 'module',
-		js: defaultModuleType
-	};
-
 	const providers = [];
 	if (Reflect.has(conf, 'babel')) {
 		try {
@@ -331,6 +326,13 @@ exports.run = async () => { // eslint-disable-line complexity
 		exit(error.message);
 	}
 
+	let moduleTypes;
+	try {
+		moduleTypes = normalizeModuleTypes(conf.extensions, defaultModuleType);
+	} catch (error) {
+		exit(error.message);
+	}
+
 	let globs;
 	try {
 		globs = normalizeGlobs({files: conf.files, ignoredByWatcher: conf.ignoredByWatcher, extensions, providers});

lib/extensions.js

@@ -2,8 +2,19 @@ module.exports = (configuredExtensions, providers = []) => {
 	// Combine all extensions possible for testing. Remove duplicate extensions.
 	const duplicates = new Set();
 	const seen = new Set();
+
+	const getArrayOfExtensions = extensions => {
+		if (Array.isArray(extensions)) {
+			return extensions;
+		}
+
+		return Object.keys(extensions);
+	};
+
 	const combine = extensions => {
-		for (const ext of extensions) {
+		const extensionsArray = getArrayOfExtensions(extensions);
+
+		for (const ext of extensionsArray) {
 			if (seen.has(ext)) {
 				duplicates.add(ext);
 			} else {

lib/module-types.js

@@ -0,0 +1,51 @@
+module.exports = (configuredExtensions, defaultModuleType) => {
+	const lockedModuleTypes = {
+		cjs: 'commonjs',
+		mjs: 'module',
+		js: defaultModuleType
+	};
+
+	const createModuleTypesFromExtensionsObject = extensionsObject => {
+		const configuredModuleTypes = {};
+
+		for (const [extension, type] of Object.entries(extensionsObject)) {
+			if (extension in lockedModuleTypes && type !== lockedModuleTypes[extension]) {
+				throw new TypeError(`.${extension} files can only be configured as ’${lockedModuleTypes[extension]}’${
+					extension === 'js' ?
+						` when the nearest parent package.json ${type === 'module' ? 'does not contain' : 'contains'} "type": "module"` : ''
+				}, found ’${type}’ instead.`);
+			}
+
+			configuredModuleTypes[extension] = type;
+		}
+
+		return configuredModuleTypes;
+	};
+
+	const createModuleTypesFromExtensionsArray = extensions => {
+		const generatedModuleTypes = {};
+
+		for (const extension of extensions) {
+			if (extension in lockedModuleTypes) {
+				generatedModuleTypes[extension] = lockedModuleTypes[extension];
+			} else {
+				generatedModuleTypes[extension] = 'commonjs';
+			}
+		}
+
+		return generatedModuleTypes;
+	};
+
+	let moduleTypes;
+	if (configuredExtensions === undefined) {
+		moduleTypes = {
+			...lockedModuleTypes
+		};
+	} else if (Array.isArray(configuredExtensions)) {
+		moduleTypes = createModuleTypesFromExtensionsArray(configuredExtensions);
+	} else {
+		moduleTypes = createModuleTypesFromExtensionsObject(configuredExtensions);
+	}
+
+	return moduleTypes;
+};