Add multi-thread support in windows systems

[kpicaza]

Is your feature request related to a problem? Please describe.
In Windows systems, we haven't a choice to install the pcntl extension, which is the easiest way to allow multi-threading execution in our context. See #13 (comment)

Describe the solution you'd like

• Develop a more robust solution for systems with the Parallel https://www.php.net/manual/en/book.parallel.php extension enabled(It will solve issues in Windows system also give a choice to use different forking strategies in nix systems),
• Document the usage and installation of PHP with ZTS and parallel ext enabled for both nix and windows

[WyriHaximus]

Build https://github.com/reactphp-parallel around ext-parallel, haven't tried it out on bare windows but happy to provide my experience for your ext-parallel experiment.