Enable socket to be closed

[andreterron]

This creates the DenoWorker.closeSocket() function, that closes the WebSocket, which can keep the process running, even when it's done executing.

Motivation

At val.town, users can sometimes write scripts that have promises that aren't awaited. We want to let those promises finish, ideally letting Deno exit whenever it thinks it's done.

Problem

Unresolved promises can cause the DenoWorker to never exit when the websocket is open. Example script:

// unresolved.js
new Promise(() => {})

When running the script above with deno run unresolved.js, it will exit naturally. When using DenoWorker, it will keep the process alive.

Solution

Closing the socket lets Deno properly exit like expected.

[stevekrouse]

Thanks for merging @KallynGowdy!!