LibJS: Add initial support for Promises

[linusg]

I figured I'd start working on Promises - still WIP. Not aiming for the full Promise API in this PR, but at least then(), catch(), finally() should be working.

The code, including some function names is closely modelled after the spec: https://tc39.es/ecma262/#sec-promise-objects

I'll probably put a checklist here and start writing tests tomorrow to see what the actual limitations currently are. Here it is:

• Promise()
• new Promise()
• Promise.prototype.then()
  • onResolve
  • onReject
• Promise.prototype.catch() (there's a catch though, pun intended - the parser doesn't understand new Promise(...).catch(...), so we have to use new Promise(...)["catch"](...) for now)
• Promise.prototype.finally()

Not sure about the non-prototype functions yet, might be out of scope for this PR:

• Promise.all()
• Promise.allSettled()
• Promise.race()
• Promise.reject() (should be relatively easy)
• Promise.resolve() (should be relatively easy)

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The following is outdated, see further down in this PR for updates.

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It can run this for example:

new Promise((resolve, reject) => resolve("foo"))
    .then(value => {
        console.log("then() received " + value);
        return value + "bar";
    })
    .then(value => {
        console.log("then() received " + value);
    });

Browser console in Firefox for comparison:

Or this, with delayed resolving (which is usually the case):

Again, Firefox for comparison:

[linusg]

Alright... almost exactly one year later, I'm happy to say this is finally done - see checklist in PR description / commit messages for details. Rebased to use the latest and greatest LibJS APIs, refactored/rewrote about half of it, added a bunch of tests, made it work in LibWeb, fixed all the bugs I know of.

Enjoy this (Lagom js) screenshot showcasing some basic functionality:

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^{I promise this is not an April Fools' joke.}

[Lubrsi]

I don't know if this should be done now, but this needs to somehow be overridable in the future for integration with HTML event loops:
https://html.spec.whatwg.org/multipage/webappapis.html#hostenqueuepromisejob

[linusg]

Definitely one for later - from what I've seen LibWeb isn't driven by an event loop at all (yet), e.g. timers just run their callbacks right away instead of queuing a task, so I just embraced that and did this: 192f3a3

Since nothing would want to subclass JS::VM (probably), we might just add a on_something function and call that if it exists, leaving queueing and therefore running the job to whatever owns the VM.