Make client more ESM-friendly (#2348)

* Use node: prefix for stdlib imports

* Make code more ESM-friendly

* Add missing mjs file

* Drop mjs file from package.json

test/integration/reporter.js

@@ -1,6 +1,6 @@
 'use strict'
 
-const assert = require('assert')
+const assert = require('node:assert')
 const { create } = require('xmlbuilder2')
 
 function createJunitReporter () {