Add xdebug tracing support

[glensc]

This uses xdebug execution_trace:

TRACE START [2020-07-30 05:48:57]
    2.7023    9969960                           -> Xhgui\Profiler\Profilers\XdebugProfiler->disable() /Users/glen/scm/xhgui/php-profiler/tests/TestCase.php:25
    4.1043    9969960                             -> xdebug_stop_trace() /Users/glen/scm/xhgui/php-profiler/src/Profilers/XdebugProfiler.php:27
    4.1044    9970016
TRACE END   [2020-07-30 05:48:58]

Probably not much usable, as seems not possible to programmatically to start profiling:

• https://stackoverflow.com/questions/8077993/can-i-manually-say-on-xdebug-profiler-to-start-profiling-in-specific-place

v3 docs:

• https://3.xdebug.org/docs/trace#functions