support emscripten build

[eagleoflqj]

Upstream patch from https://github.com/fcitx-contrib/fcitx5-js

[wengxt]

I don't like having an external file dependency.

I think we could somehow, create a stub for EventLoop and having a global factory class to allow external event loop implementation.

[eagleoflqj]

I have no idea how to achieve that with a factory.
Added the event_js.cpp, since there is nothing specific to fcitx5-js. emscripten_async_call is generic API https://emscripten.org/docs/api_reference/emscripten.h.html#c.emscripten_async_call

[wengxt]

Without breaking the API, here's what you can do.

1. create a stub only EventLoop that takes a EventLoopImpl as parameter.
2. Create a global factory callback that allow external code to override the creation of EventLoopImpl.
3. Before initialize Instance, set the factory function

In other words, use Dependency injection to make your js eventloop implementation live outside of the main repo.

[eagleoflqj]

Updated.

[eagleoflqj]

See fcitx-contrib/fcitx5-js#14 for downstream build.