add @mocha package declaration for mocha.loadFilesAsync() (#42684)

* add declaration for Mocha.loadFilesAsync()

* add test for mocha.loadFilesAsync()

types/mocha/index.d.ts

@@ -231,6 +231,13 @@ declare class Mocha {
      */
     run(fn?: (failures: number) => void): Mocha.Runner;
 
+    /**
+     * Loads ESM (and CJS) test files asynchronously.
+     *
+     * @see https://mochajs.org/api/mocha#loadFilesAsync
+     */
+    loadFilesAsync(): Promise<void>;
+
     /**
      * Load registered files.
      *

types/mocha/mocha-tests.ts

@@ -833,6 +833,10 @@ function test_browser_mocha_setup_all_options() {
     });
 }
 
+function testLoadFilesAsync() {
+    mocha.loadFilesAsync();
+}
+
 function test_constructor_slow_option() {
     const m: Mocha = new LocalMocha({ slow: 25 });
 }