v4: Generate API and Gateway structures

[diamondburned]

Ideally, all of our API and WS gateway data structures should be autogenerated. This will make maintenance overhead so much lower.

Plan

We will use discord-api-types's existing type definitions for this, rather than trying to manually parse the documentation ourselves. A lot of breaking changes may happen because of this, which is why it's part of the v4 milestone.

A very easy way to consume the documentation is by using Deno: deno doc --json https://deno.land/x/discord_api_types/v10.ts. A lightweight code generator can accept the Deno-generated JSON representation of the TypeScript file as input.