debug_get_callstack for HTML5

This extension corrects a minor oversight being that debug_get_callstack on GameMaker's HTML5 target will not return the original script names despite the required data being stored for script_get_name.

That will probably get fixed by late 2019.

GMS2 instructions: import the extension (right-click on Extension in resource tree, pick "import", pick the GMEZ). That's it.

GMS1 instructions: import the extension (ditto) and then import the "gmcallback_debug_get_callstack_js" script from it. Or create a blank script named so.