Draft: XDebug support via dlopen

[adamziel]

What is this PR doing?

Explores support for dynamic loading of PHP extensions.

Supersedes #673, consult that PR for the progress history and helpful comments.

Related to #314, #655, #89

Minimal dlopen example without PHP involved.

Testing instructions

Clone this branch to a directory with path /Users/cloudnik/www/Automattic/core/plugins/playground-2/wordpress-playground. It's not great, I know. I want to remove the explicit path dependency soon.

Then, create a /usr/local/etc/php.ini file with the following contents:

allow_url_fopen=1
extension=/Users/cloudnik/www/Automattic/core/plugins/playground-2/wordpress-playground/packages/php-wasm/node/public/mbstring.so
zend_extension=/Users/cloudnik/www/Automattic/core/plugins/playground-2/wordpress-playground/.libs/xdebug.so
xdebug.mode=debug
sys_temp_dir=/tmp

Finally, run:

$ nx reset; PHP=8.2 nx start php-wasm-cli -z ./.libs/xdebug.so ./test.php

Here's what you should see:

Remaining work

• Reconcile this branch with trunk. This PR ships many stale files.
• Clean up the code
• Make this work with any disk path, not just the one listed above
• Make XDebug step debugger work

[adamziel]

This PR is now much simpler and xdebug functions work! However, connecting to a step debugger doesn't work yet. It seems like xdebug isn't even trying to open a remote socket here:

https://github.com/xdebug/xdebug/blob/49afefb7d9d313300369775b110c2ca2f2410b71/src/debugger/com.c#L246-L257

Xdebug: [Step Debug] Could not connect to debugging client. Tried: 127.0.0.1:9003 (through xdebug.client_host/xdebug.client_port).