[Windows][MSVC] Add support of std::initializer_list<string_view> for windows::default_launcher::operator()

Got a problem at Boost version 1.88

This can be compiled
```
#define WIN32_LEAN_AND_MEAN
#include "boost/process.hpp"

int main()
{
boost::asio::io_context ctx;

auto process = boost::process::process(ctx, "C:\\Windows\\System32\\cmd.exe", {});

return 0;
}
```

And this can't
```
#define WIN32_LEAN_AND_MEAN
#include "boost/process.hpp"

int main()
{
boost::asio::io_context ctx;
boost::process::default_process_launcher launcher;

auto process = launcher(ctx, "C:\\Windows\\System32\\cmd.exe", {});

return 0;
}
```

Because the basic_process constructor supports std::initializer_list, but the windows::default_launcher::operator() does not.
I would like this functionality to be implemented by the developers/community.