feat(cli): add ESM config loader

This is a conventional 'best-effort' approach for loading ESM modules
dynamically and dealing with the asynchronicity.

With some refactoring of the configuration loader logic, instead
`lib/nodejs/esm-utils.js::requireOrImport` could be reutilized deduplicating any
logic in regards to handling ESM. ESM support in Node.js is stable and I doubt
there will be any major changes to the way it is supported, but still,
deduplication is always nice, and there might be some slight adjustments to the
Node.js module resolution algorithm (https://nodejs.org/api/esm.html#resolution-algorithm)

Implements: #5049

Author: ByteB4rb1e

lib/cli/config.js

@@ -24,6 +24,7 @@ const utils = require('../utils');
 exports.CONFIG_FILES = [
   '.mocharc.cjs',
   '.mocharc.js',
+  '.mocharc.mjs',
   '.mocharc.yaml',
   '.mocharc.yml',
   '.mocharc.jsonc',
@@ -49,6 +50,28 @@ const parsers = (exports.parsers = {
       return require(filepath);
     }
   },
+  esm: filepath => {
+    let importer;
+
+    try {
+        /**
+         * @see
+         * {@link https://tc39.es/ecma262/multipage/ecmascript-language-scripts-and-modules.html#sec-imports
+         *  | ECMAScript Specifications}
+         */
+        importer = new Function('spec', 'return import(spec);');
+    } catch(_) {
+        throw new Error(
+            `unable to load ${filepath}. ` +
+            'Your current runtime does not support dynamic module imports. ' +
+            'Please define the configuration with another file type.'
+        );
+    }
+
+    // fail hard, in case the environment is unable to import the ESM module so
+    // that the error message is as verbose as possible
+    importer(filepath);
+  },
   json: filepath =>
     JSON.parse(
       require('strip-json-comments')(fs.readFileSync(filepath, 'utf8'))
@@ -73,6 +96,8 @@ exports.loadConfig = async filepath => {
       config = parsers.yaml(filepath);
     } else if (ext === '.js' || ext === '.cjs') {
       config = parsers.js(filepath);
+    } else if (ext === '.mjs') {
+      config = await parsers.esm(filepath);
     } else {
       config = parsers.json(filepath);
     }