Fixes #87

The main changes :

• simpletest+pear/textdiff dev dependencies are removed
• phpunit added with its version depending of PHP version
• workfllow matrix extended to PHP8.3
• test files are divided in 3 parts :
  • a css file to test
  • a settings php file
  • an expected file with a php or a css content

The csst files division helped me during the migration. Then, I realized it was easier to check and debug with a nice syntax highlighting in my IDE. Therefore, I propose to keep the files divided

Before migration, 132 tests were passing
After migration, with the same number of tests, 136 tests are passing. I guess there was homonymous test names...

Minor changes :

• the README file is "corrected" in order to generate fewer errors with a markdown parser
• keywords were added in composer.json to help users to find this package in packagist.org