Implement Wasm support

[SamRodri]

Winit supports web, so maybe we can extend zng-view to provide a quick implementation of this.

This platform enables interactive examples in docs.

This platform enables some support for all operating systems that provide a web view.

[SamRodri]

Idea, instead of loading Webrender to render to canvas we could convert the display list to Html elements, this might be more performant. This might also lets us use ARIA for accessibility.

[SamRodri]

Since #391 the "wasm-unknown-unknown" target now compiles without error. Unfortunately many dependencies compile but panic during runtime so WASM support will be released gradually.

Next task is running all app extensions (APP.defaults()) in headless without renderer.

Next major task is implementing a basic zng-view-web.

[SamRodri]

Wasm is to limited to support at this time, many dependencies build but panic, including Rust's std. My Initial expectation was that only some adaptations would be required to implement the zng-view-web backend.

I will work on mobile platforms first and return to wasm only when those are implemented.