Public API to get `WebAssembly.Exception` `val` from `exception_ptr`

It would be useful to have a public (stable) API to convert a `std::exception_ptr` (or at least the current exception) to an `emscripten::val` for a `WebAssembly.Exception` (or `Error` derivative for Emscripten EH). For example, to invoke a JS on-error callback when an unexpected C++ exception occurs.

Currently there is `val::take_ownership(emscripten::internal::_emval_from_current_cxa_exception())`, but this is an internal API and is marked as C++20 only because it's used for coroutines.