Add PHPUnit and Codecov badges to README

Enhanced the README by adding badges for PHPUnit and Codecov. This makes it easier for users to quickly check the test status and code coverage metrics directly from the README page. The badges are linked to the respective test results and metric reports for immediate reference.

README.md

@@ -1,4 +1,6 @@
 # PHP AST Check Diff Tool
+[![phpunit](https://github.com/o0h/php-ast-check-diff/actions/workflows/phpunit.yml/badge.svg)](https://github.com/o0h/php-ast-check-diff/actions/workflows/phpunit.yml)
+[![codecov](https://codecov.io/gh/o0h/php-ast-check-diff/graph/badge.svg?token=hxtB6lwmK5)](https://codecov.io/gh/o0h/php-ast-check-diff)
 
 ## Overview