fix: JS API shouldn't require reference to window object

[niloc132]

Also introduces a dh.Event type to use instead of the browser Event and CustomEvent types.

Fixes #5537
Fixes #6190

[bmingles]

I've tested this in the vscode extension by removing most of the polyfill we needed before. Remaining polyfill is:

import ws from 'ws';

export function polyfillDh(): void {
  /* @ts-ignore */
  global.self = global;
  /* @ts-ignore */
  global.this = global;

  global.WebSocket = ws as unknown as (typeof global)['WebSocket'];
}

I think we can get rid of the global.self + global.this assignments by using globalThis in js api.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/globalThis

@mofojed any opinions on this?

[bmingles]

Do we have to use Promise.resolve in the return? I thought non-promise and promise returns from a then both result in the same thing.

[niloc132]

In JS yes, but in Java you cannot define a method signature to return two unrelated types - union types effectively don't exist in this fashion (and while technically jvm bytecode allows method signatures to be specified based on return type, the java language doesn't have support for that). So, when we want to return a value directly, we must always wrap in Promise.resolve(...).

[bmingles]

LGTM.