Custom stack traces

[Aidan63]

See HaxeFoundation/haxe#12213

This updates hxcpp unhandled exceptions to only call __hx_dump_stack if the caught object does not have a __customStack field, in which case that is printed out instead.

[Simn]

Hmm... the HL implementation is stricter in the sense that it actually checks if we're looking at an instance of haxe.Exception. Though now I wonder if that's necessary. I'm generally wary of magic field names like this but if we add one I really think it should be something leading with _hx_. We don't really reserve anything like __customStack in general, so if someone had such a field with a different type we run into trouble here.

[Aidan63]

Good point, I've changed it to _hx_customStack the field on the cpp haxe exception class will need updating. I'm going to bump up looking at revamping hxcpps exception stuff on my internal todo list, so hopefully it won't be around for too long.