Initial commit of xdebug

Summary: Initial commit of xdebug, fixes T2479.

Test Plan:
Installed it and checked the extension via phpinfo();
Using visual studio code, run a debugging session with a simple php file.

Reviewers: Triage Team, DataDrake

Reviewed By: Triage Team, DataDrake

Subscribers: alexandruchircu, Justin, ikey, stobbsm, DataDrake

Maniphest Tasks: T2479

Differential Revision: https://dev.solus-project.com/D340