Emscripten implementation

[jeroen]

Can we mimic V8 with the emscripten API for wasm workers (one for each context) and emscripten_run_script_string?

• https://emscripten.org/docs/api_reference/wasm_workers.html
• https://emscripten.org/docs/api_reference/emscripten.h.html#c.emscripten_run_script_string