ClassMapGenerator: stabilize the heredoc/nowdoc stripping
ClassMapGenerator: stabilize the heredoc/nowdoc stripping

I've looked into 10067 and have come to the conclusion that using a single regex to strip the heredoc/nowdocs is always going to run into trouble as:
* Either the matching will be too greedy (issue 10067);
* Or the matching will run into backtrace limits for large heredoc/nowdocs.

We cannot solve both within a single regex.

So, I'm proposing a slightly different solution which should support both and should also improve performance for files containing large heredoc/nowdocs.

The `stripHereNowDocs()` function will find a start marker and remember the offset of the start marker.
It will then find the end marker and strip the contents between the two (replace with `null`).
The function will then recurse onto itself until all heredocs/nowdocs in a file have been removed.

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