feat: support loading ESM config files

In Node.js v24.0.0, v22.12.0, and v20.19.0, ECMAScript modules can be
loaded via `require` in a CommonJS context without an experimental flag.
When a system already uses ESM for the majority of its code it can be
inconsistent and confusing to have a single CommonJS file.

This change allows n-test to load a config file that's an ECMAScript
module by looking for the default export. We only try the `default`
property of the imported config file if it's not an array.

See https://nodejs.org/api/modules.html#loading-ecmascript-modules-using-require
for more information on how this works and why we use `__esModule` and
the `default` property.

Author: rowanmanning

lib/smoke/smoke-test.js

@@ -52,7 +52,10 @@ class SmokeTest {
 	}
 
 	async compileTests (sets) {
-		const configsToRun = await filterConfigs(this.configFile, sets, this.isInteractive);
+		let configsToRun = await filterConfigs(this.configFile, sets, this.isInteractive);
+		if (!Array.isArray(configsToRun) && configsToRun.__esModule) {
+			configsToRun = configsToRun.default;
+		}
 		const canRunCrossBrowser = await getBrowserstackConfig(this.enabledBrowsers);
 		const globalOptions = {
 			headers: this.globalHeaders,