fixes #1948 replace cssmin by minify

- switched from cssmin to matthiasmullie/minify for CSS minification.
- the minify lib requires the path-converter lib (from the same author), which has been added as well (in include/minify/path-converter).

This change is necessary because cssmin is not compatible with PHP 8.4

include/minify/src/Exception.php

@@ -0,0 +1,20 @@
+<?php
+
+/**
+ * Base Exception.
+ *
+ * @deprecated Use Exceptions\BasicException instead
+ *
+ * @author Matthias Mullie <minify@mullie.eu>
+ */
+
+namespace MatthiasMullie\Minify;
+
+/**
+ * Base Exception Class.
+ *
+ * @deprecated Use Exceptions\BasicException instead
+ *
+ * @author Matthias Mullie <minify@mullie.eu>
+ */
+abstract class Exception extends \Exception {}