Replace included wasm_exec.js with the local one

[nicolanrizzo]

The WebAssembly JavaScript support file is currently included in the project. I think a better approach is using the file present in the Go root folder.

According to the WebAssembly documentation, wasm_exec.js can be retrieved from the local Go root folder using

cp "$(go env GOROOT)/misc/wasm/wasm_exec.js" .

Therefore in this commit:

• I have removed frontend/js/wasm_exec.js
• I have included this file to the .gitignore to avoid risking to commit it
• I have added the WebAssembly documentation command to the build script