Workaround for legacy newsletter editor tests after updating chai

Chain newly has to be imported as module and doesn't support CommonJs
[MAILPOET-6008]

mailpoet/tests/javascript-newsletter-editor/mocha-chai.mjs

@@ -0,0 +1,8 @@
+import * as chai from 'chai';
+import sinonChai from 'sinon-chai';
+import chaiJq from 'chai-jq';
+
+chai.use(sinonChai);
+chai.use(chaiJq);
+
+global.expect = chai.expect;