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/Exceptions/PatternMatchException.php

@@ -0,0 +1,36 @@
+<?php
+
+/**
+ * Pattern match exception.
+ *
+ * Please report bugs on https://github.com/matthiasmullie/minify/issues
+ *
+ * @author Ere Maijala <ere.maijala@helsinki.fi>
+ * @copyright Copyright (c) 2012, Matthias Mullie. All rights reserved
+ * @license MIT License
+ */
+
+namespace MatthiasMullie\Minify\Exceptions;
+
+/**
+ * Pattern Match Exception Class.
+ *
+ * @author Ere Maijala <ere.maijala@helsinki.fi>
+ */
+class PatternMatchException extends BasicException
+{
+    /**
+     * Create an exception from preg_last_error.
+     *
+     * @param string $msg Error message
+     */
+    public static function fromLastError($msg)
+    {
+        $msg .= ': Error ' . preg_last_error();
+        if (PHP_MAJOR_VERSION >= 8) {
+            $msg .= ' - ' . preg_last_error_msg();
+        }
+
+        return new PatternMatchException($msg);
+    }
+}