misc/wasm: add scripts for running WebAssembly binaries
This commit adds scripts for running the WebAssembly binaries that the
Go compiler will produce.
The script go_js_wasm_exec uses Node.js to run the binaries. Adding it
to PATH will enable "go run" and "go test" to work for js/wasm
without having to manually provide the -exec flag.
See https://golang.org/cmd/go/#hdr-Compile_and_run_Go_program
for more information.
The web page wasm_exec.html is an example on how to run the same
binaries in a web browser.
Both scripts use wasm_exec.js as a shared library.