Main executor for WASM doesn't seems to be fully implemented

[keminming]

By looking at
Main_executor.hpp, for Emscripten case, if seems falls back to use default_executor. To my knowledge jobs will be run in secondary threads in default_executor. So need main executor guarantee the job will be scheduled to the main thread sharing the semantic for other platform.

#elif STLAB_MAIN_EXECUTOR == STLAB_MAIN_EXECUTOR_EMSCRIPTEN
using main_executor_type = default_executor_type;

Also. #elif STLAB_MAIN_EXECUTOR == STLAB_MAIN_EXECUTOR_PORTABLE
#if APPLE not define, the implementation is not complete

[dabrahams]

Contributions welcome.