Common JS shell infrastructure

cc @jfbastien @saambarati @kmiller68 @binji @MikeHolman

JF brought up the issue that we'll want a shared system in the JS shells to be able to test wasm threaded code. (And JS code for that matter - everyone will love us.)

To kick us off, I propose the following strawman:

The main thread can create new workers:

var w = new Worker(text)  // where text is defined below
w.postMessage(obj)           // what you think
w.onmessage = function (ev) { ... } // where ev.data is something sent by postMessage
dispatch()                           // enter the event loop

The worker can then:

onmessage = function (ev) { ... } where ev.data is something sent by postMessage
postMessage(obj)       // what you think

Note,

• There is an implicit event loop in the worker.
• There is an explicit event loop in the main thread by means of dispatch(); when it returns the main thread continues and may exit. (Debatable how good this explicit event loop is, not sure what d8 does here. Discuss.)
• The worker cannot create new workers (keeps things simple).
• The object to be posted is anything structured-cloneable
• I haven't bothered to worry about transfering objects here

The "text" for a worker is a string that starts with 'text/plain;', this is for web compat so that tests can just carry over (ideally).