Add `Promise` version of events

[kazarmy]

This pr extends the generator script to add Promise versions of events, e.g.:

domContentEventFired(): Promise<Protocol.Page.DomContentEventFiredEvent>;

as a complement for

on(event: 'domContentEventFired', listener: (params: Protocol.Page.DomContentEventFiredEvent) => void): void;

---

The motivation for this pr is to provide typings support for the following syntax:

const {Page} = client;
await Page.domContentEventFired();

Currently, typings support is provided for the following syntax via DefinitelyTyped's chrome-remote-interface typings:

await client['Page.domContentEventFired']();

but it is less elegant and the client part shouldn't really be necessary. Unfortunately, it doesn't seem possible to provide typings for the former syntax via protocol-mapping.d.ts since it is not possible to split something like 'Page.domContentEventFired' into its component parts with the current version of TypeScript (4.3).

---

In the code, there are two forEach loops for emitApiEvent() so that all the on() overloads are kept together.

Fixes #246.