Add xterm.js in shell command

As we've now added shell command, It would be a nice feature enhancement to access shell in browser. It would be useful when we don't have access to our usual dev environment and would like to access shell of website anyways.

It should be included in admin tools like `example.com/ee-admin/webshell`.

We might need a node server script to handle backend of xterm frontend in browser. A demo is already included in xterm which can be used (mostly) unchanged.