CI: add a prettier GitHub action to format code automatically (#988)

* CI: add a prettier GitHub action to format code automatically

* improve GitHub Action config and format some files

* Apply formatting changes

* CI: make the prettier action a standalone action

* Apply formatting changes

* CI: add push as new trigger event

Co-authored-by: Mark-Fenng <Mark-Fenng@users.noreply.github.com>

packages/rrweb/test/integration.test.ts

@@ -667,7 +667,7 @@ describe('record integration tests', function (this: ISuite) {
     const snapshots = await page.evaluate('window.snapshots');
     assertSnapshot(snapshots);
   });
-  
+
   // https://github.com/webcomponents/polyfills/tree/master/packages/shadydom
   it('should record shadow doms polyfilled by shadydom', async () => {
     const page: puppeteer.Page = await browser.newPage();