Support omitting optional arguments in js embind bindings

[synoet]

Please include the following in your bug report:

Version of emscripten/emsdk:

emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 3.1.55 (165133b1cc977f0b9a277e42ef809b823157189c)
clang version 19.0.0git (https:/github.com/llvm/llvm-project 6c7805d5d186a6d1263f90b8033ad85e2d2633d7)
Target: wasm32-unknown-emscripten
Thread model: posix

Issue
When using optional arguments with embind, it would be more intuitive and convenient to allow omitting the optional arguments entirely, rather than requiring passing undefined for the optional parameters.

inline std::string testOptional(std::string str, std::optional<std::string> optionalString)
{
}

EMSCRIPTEN_BINDINGS(test)
{
    register_optional<std::string>();
    function("test", &testOptional);
}

I would expect the generated JavaScript bindings to allow calling the function like this:

testOptional("someString")

Instead of requiring the following:

testOptional("someString", undefined)

I noticed a comment on the original optional implementation pull request (#21076) mentioning plans to support this feature.

[synoet]

Thanks!