ctx.store_exception_stack is always disabled for custom js generator

Its initial value is defined in `alloc_ctx` (genjs.ml)
as
`store_exception_stack = if Common.has_dce com then (Common.has_feature com "haxe.CallStack.exceptionStack") else List.exists (function TClassDecl { cl_path=["haxe"],"CallStack" } -> true | _ -> false) com.types;`
But for some odd reason it does not affect custom js generator, despite that `alloc_ctx` is called by `MacroApi.set_js_generator`

If for some reason it cannot be fixed, maybe you can store and use `store_exception_stack` as feature, so it can be overriden from custom generator?
Or at least expose it via JSGenApi?
Thanks.