Implemented xdebug remote commands

Summary: This diff implements the following remote commands:
* stop, detach
* stack_get, stack_depth
* context_names, typemap_get
* source
* stdout, stderr
* xcmd_profiler_name_get

The description of each is right above the implementation, so I won't go
into detail here.

These commands were all very simple to implement, which is why they are done
in this diff. The rest are incoming.

Other changes of note:
* XDebugCommand construction is no longer const. This was limiting the use of
  `XDebugCommand::handleImpl` and wasn't really necessary.
* `XDebugCommand::handleImpl` doesn't return a bool any longer, as it was the
  same result for almost all commands

Reviewed By: @ptarjan

Differential Revision: D1511176