Implement automatic CSS minify (build process)

[Zodiac1978]

The files admin.min.css should be auto generated on release.

Additionally we should apply some sort of cache busting for the CSS file. For example append the plugin version, so that the new file is loaded if changed.

https://github.com/Zodiac1978/tl-template-checker/blob/master/child-theme-check.php#L103-L107

Additionally the minified version should only be used if SCRIPT_DEBUG is false, otherwise the non-minified version should be used:
https://wordpress.org/support/article/debugging-in-wordpress/#script_debug