ClassMapGenerator: fix the regex
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

nicolas-grekas/composer,alcohol/composer,milleruk/composer,stof/composer,johnstevenson/composer,staabm/composer,composer/composer,stefangr/composer