XSLTProcessor recursion limit

Hi internals Based on https://bugs.php.net/bug.php?id=71571 I've opened a PR to implement two new properties for XSLTProcessor: maxTemplateDepth and maxTemplateVars. The reasoning behind this is that large templates with lots of recursion and/or variables can bump against the default recursion limit set by the libxslt library. PR link: https://github.com/php/php-src/pull/13731 I used an adapted version of https://github.com/nikic/popular-package-analysis to download every package that satisfies the search term "xsl". Then I checked if there are any classes that extend XSLTProcessor and have the maxTemplateDepth or maxTemplateVars field. None do, and as such no package in packagist will observe a BC break because of this PR. One sad detail however, is that you can set the recursion limit so high that you can exhaust the stack space, which will crash the process with a segfault. In fact, you can hit this already right now without the PR, using XSL. I.e. you can create a very deep VM re-entry that won't cause the stack limit protection to kick in, and then start a recursive XSL template processing that does not hit XSL's limit, but exhausts the remaining stack space. Note that as soon as you do callbacks however, the stack limit protection _will_ kick in. I tried to check if it's possible to prevent this, but the stack limit check would need to happen inside the libxslt library, so I don't think it's possible. Let me know if there are any complaints about this. If no complaints, I'll merge this in 2 weeks or so. Kind regards Niels