Bump PHP version and add support to GitHub Actions

[sarjona]

Bumped PHP version to 7.3.0 and PHPUnit version to 9.5.* to meet Moodle requirements.
Apart from that, for now, the sabberworm/php-css-parser version has been fixed to 8.1.* because 8.2 onwards has a bug with
comments (raised by a couple of PHPUnit tests).

Besides, Github Actions support has been added to RTLCSS for PHP, using the recommended configuration.

The result of the GHA jobs in my local repository can be checked in https://github.com/sarjona/rtlcss-php/actions/workflows/ci.yml

I've also added the image in the README.md file (which won't work until the GHA job will be run by the first time).

[stronk7]

Nice fixes!

Some little comments, copied from elsewhere:

1. Only detail I can imagine is why in GHA you “restrict” the phpunit execution to tests/MoodleHQ/RTLCSS/RTLCSSTest.php . Given it has a phpunit.xml, it should be able to find all the tests without specifying any.
2. Also the matrix.extensions seems to be unused (surely also the max_input_vars). They are only needed for moodle, so far.
3. And a Fixes #3 anywhere in the commit message will, automatically, close Unit tests won't run #3
4. Finally, surely after all the latest fixes and this... we'll need to do a 1.0.1 release or so (I imagine). To be incorporated to core.

Ciao :-)

[sarjona]

Hi @stronk7 !
Thanks for reviewing the patch! Good catches!!

1. Removed tests/MoodleHQ/RTLCSS/RTLCSSTest.php, so now any PHPUnit test should be executed.
2. matrix.extensions and max_input_vars removed because you were right, they weren't used.
3. Added Fixes Unit tests won't run #3 to the commit message (I wasn't aware of that... or didn't remember it!)
4. I was planning to release a version once this will be merged and then upgrading the library in core using MDL-72626.

[stronk7]

Sold!