add wasm support

[igagis]

[igagis]

basic emscripten support added.

Still need to add:

• mouse cursor management (see https://github.com/Armchair-Software/emscripten-browser-cursor)
• dynamic main loop timing mode, do not run in busy loop if there is nothing to update

[igagis]

if emscripten_set_main_loop() is set up with very low fps, then it will not invoke the loop callback immediately when there is any input event, e.g. mouse or key. Instead it will invoke it in accordance with requested fps.
So, to implement the dynamic main loop timing mode it is needed to subscribe to all the needed events and in handler callbacks invoke the main loop out of order.