[Feature request] [Discussion] Document/legalize Embind support for custom marshalling

I have a C++ class which I want to (un)marshall to JS in a custom way. There is an "unofficial" way of doing so which works at the moment: provide specializations for `emscripten::internal::{BindingType,TypeId}` in each translation unit where you want them (together with `emscripten/bind.h`), like this:

```
namespace emscripten::internal {
struct BindingType {
typedef internal::EM_VAL WireType;
static WireType toWireType(const SomeWeirdType& foo) {
// One can also call JS-side functions here and let them construct the new object instead of crafting it here.
return BindingType::toWireType(val(foo.a * 10 + foo.b));
}
};

struct TypeID {
static constexpr TYPEID get() {
return LightTypeID::get();
}
};

struct TypeID {
static constexpr TYPEID get() {
return LightTypeID::get();
}
};

struct TypeID {
static constexpr TYPEID get() {
return LightTypeID::get();
}
};

struct TypeID {
static constexpr TYPEID get() {
return LightTypeID::get();
}
};
}
```

However, this is not documented anywhere. **Is it likely for this API to change rapidly?** If not, it would be nice to have some documentation and, possibly, legalization of such activities (at the very least, moving it out of `internal` namespace).

**How about establishing a documented custom marshalling API?**

Some example usecases are:

1. `std::function` which I want to convert to JS `function` (forgetting about ownership and leaks) to provide callbacks to JS easily.
2. Wrapper class around `HTMLElementDocument` or some other JS api which I want to call from C++ for whatever reason instead of writing JS glue code. Say, it holds a reference to `emscripten::val` and just redirects calls to `val::operator()`. Something like Rust's [`wasm-bindgen`'s `web_sys`](https://rustwasm.github.io/wasm-bindgen/api/web_sys/struct.HtmlElement.html).
3. Custom `Promise`-like class which I want to interop with JS, so I cannot just directly bind properties/methods, hence I have to create a new object on JS side in a specific way (probably like in Rust's [`wasm_bindgen_futures`](https://rustwasm.github.io/wasm-bindgen/api/wasm_bindgen_futures/index.html), but I haven't looked into it).