ClassMapGenerator: fix the regex

By using a look ahead assertion to match "new line - maybe whitespace - marker", the negative performance impact of the `.*` is significantly mitigated and backtracing will be severely limited.

This fixes the bug as reported in 10037.

The bug was discovered due to a PHP 8.1 "passing null to non-nullable" deprecation notice being thrown, but is not a PHP 8.1 bug.

In actual fact, this issue affected all PHP versions and could lead to incomplete classmaps when the code base contained files with huge heredocs/nowdocs.

The regex change (not completely) incidentally also fixes an issue with markers in a heredoc/nowdoc not being correctly handled. This bug could lead to "classes" being added to the class map which aren't actually classes.

Fixes 10037

src/Composer/Autoload/ClassMapGenerator.php

@@ -246,7 +246,7 @@ class ClassMapGenerator
         }
 
         // strip heredocs/nowdocs
-        $contents = preg_replace('{<<<[ \t]*([\'"]?)(\w+)\\1(?:\r\n|\n|\r)(?:.*?)(?:\r\n|\n|\r)(?:\s*)\\2(?=\s+|[;,.)])}s', 'null', $contents);
+        $contents = preg_replace('{<<<[ \t]*([\'"]?)(\w+)\\1(?:\r\n|\n|\r)(?:.*(?=[\r\n]+[ \t]*\\2))[\r\n]+[ \t]*\\2(?=\s*[;,.)])}s', 'null', $contents);
         // strip strings
         $contents = preg_replace('{"[^"\\\\]*+(\\\\.[^"\\\\]*+)*+"|\'[^\'\\\\]*+(\\\\.[^\'\\\\]*+)*+\'}s', 'null', $contents);
         // strip leading non-php code if needed