Add test from example at PostHog/posthog#21427

Author: eoghanmurray

packages/rrweb-snapshot/test/css.test.ts

@@ -119,6 +119,16 @@ describe('css parser', () => {
     expect(out3).toEqual('[data-aa\\:other] { color: red; }');
   });
 
+  it('parses nested commas in selectors correctly', () => {
+        const result = parse(
+          `
+body > ul :is(li:not(:first-of-type) a:hover, li:not(:first-of-type).active a) {
+  background: 'red';
+}
+`);
+    expect((result.stylesheet!.rules[0] as Rule)!.selectors!.length).toEqual(1);
+  });
+
   it('parses imports with quotes correctly', () => {
     const out1 = escapeImportStatement({
       cssText: `@import url("/foo.css;900;800"");`,