ESM: Enable dynamic imports.

This PR enables dynamic imports.

Dynamic imports are a special syntax that is function-like and returns a Promise.

import * as system from 'system';

import('./x.js').then((module) => {
    log(module.default);
}).catch(err => {
    logError(err);
}).finally(() => {
    imports.mainloop.quit();
});

imports.mainloop.run();

Dynamic imports also provide inter-op with legacy GJS scripts...

const { GLib } = imports.gi;

var MyApp = ...

import('./module.js').then((module) => {
    log(module.run(MyApp));
}).catch(err => {
    logError(err);
});