WebAssembly: Wasi Support

The WebAssembly System Interface - https://wasi.dev

Wasi covers some of the same ground as emscripten, but is less fully featured. Its subcomponents are wasi-sdk, which provides the clang C/C++ compiler; and the wasmtime runtime (libc, libstd++).

Wasi is interesting since it runs on many operating systems, and is designed to be an embeddable sandbox. There is no mandatory JavaScript usage.

Qt support can be developed gradually (Qt Core first), in parallel with the emscripten target.