Support for await and async

[velara3]

The description from MDN:

The await operator is used to wait for a Promise and get its fulfillment value. It can only be used inside an async function or at the top level of a module.

Await is useful for calling and waiting for results from remote services and code readability.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/await#top_level_await

[hydroper]

Since ECMAScript 4 proposed generators without using a * prefix in function names unlike in ECMAScript Harmony, I suggest, please... maybe do not introduce the async modifier and comply with ECMAScript 4?

Not that it's difficult to parse, but it'd be nice to have a general consensus in the syntax, since ActionScript 3.0 implements most of ECMAScript 4.

ES4 examples:

/* methodThatYields(): Iterator.<Number> */

function methodThatYields(): Number {
    for (var i = 0; i < 10; i++) {
        yield i
    }
}

Example of asynchronous methods as in AS3Parser:

function asynchronousMethod1(): Promise.<Number> (
    Promise.resolve(10)
);
function asynchronousMethod2(): Promise.<Number> {
    return await asynchronousMethod1();
}

I've an implementation of Promise in my repository, as well (based on a polyfill), but I added a simple type parameter, so you may want to look at agera.util instead.