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>

CONTRIBUTING.md

@@ -1,13 +1,17 @@
 # Contributing to rrweb
+
 We want to make contributing to this project as easy and transparent as
 possible.
 
 ## Our Development Process
+
 The majority of development on rrweb will occur through GitHub. Accordingly,
 the process for contributing will follow standard GitHub protocol.
 
 ## Pull Requests
+
 We actively welcome your pull requests.
+
 1. Fork the repo and create your branch from `master`.
 2. If you've added code that should be tested, add tests
 3. If you've changed APIs, update the documentation.
@@ -15,10 +19,12 @@ We actively welcome your pull requests.
 5. Make sure your code lints and typechecks.
 
 ## Issues
+
 We use GitHub issues to track public bugs. Please ensure your description is
 clear and has sufficient instructions to be able to reproduce the issue.
 
 ## License
+
 rrweb is [MIT licensed](https://github.com/rrweb-io/rrweb/blob/master/LICENSE).
 
 By contributing to rrweb, you agree that your contributions will be licensed