Add a WASM-sandboxed JS executor.

After this MR, several problems remain:

• we are not linting the javascript-wasmedge files because the lint rules don't allow ES modules;
• the JS WASM executor can't yet handle UTF;
• the JS WASM executor doesn't do Errors or stderr
• the benchmark for the JS WASM executor fails mysteriously

I have filed Phabricator tasks to address these.