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/IOException.php

@@ -0,0 +1,20 @@
+<?php
+
+/**
+ * IO Exception.
+ *
+ * Please report bugs on https://github.com/matthiasmullie/minify/issues
+ *
+ * @author Matthias Mullie <minify@mullie.eu>
+ * @copyright Copyright (c) 2012, Matthias Mullie. All rights reserved
+ * @license MIT License
+ */
+
+namespace MatthiasMullie\Minify\Exceptions;
+
+/**
+ * IO Exception Class.
+ *
+ * @author Matthias Mullie <minify@mullie.eu>
+ */
+class IOException extends BasicException {}