Add TypeScript definition; allow url argument to be an async function

[finom]

The PR consists two changes:

• TypeScript support. This is a today's must-have.
• Async function to be supported as URL argument. The reason is that some APIs require to request a renewable key to be used as part of WS URI. For example Binance API requires a POST request to be made to get listenKey string. This is what the code looks like (pasted from a project I work on) after the change:

  const stream = new RobustWebSocket(async function getUrl(): Promise<string> {
      try {
        const { listenKey } = await api.futuresUserDataStream('POST');
        return `${options.accountStreamURL}${listenKey}`;
      } catch (e) {
        console.error(e);
        await delay(5_000);

        return getUrl();
      }
    });

I have published the change on NPM to use the library while this PR is waiting to be approved: https://www.npmjs.com/package/altamoon-robust-websocket.

Thank you for your work!

[nathanboktae]

Thanks for the PR. Happy to take the TypeScript definition file.

However, for the async changes, it a) forces the function to be async always, and not allow sync; this is a breaking change. b) requires ESNext, as currently this library is vanilla ES5.

You can fix both by allowing getUrl to return string | Promise<string> and use standard Promise APIs. If you do that, please update the documentation, add a test, and I'll be glad to take the PR.

[finom]

@nathanboktae thank you for quick response. Unfortunately that's a lot of work and I we don't have time allocated on that, sorry. We're going to use the fork 😬 But feel free to borrow our changes 😅