Implement promises library with non-blocking methods

[tmalahie]

This PR adds an alternate version of the wrapper library that supports async operations.
The issue with the current wrapper is that each method is blocking, which is generally an anti-pattern in JS. Concretely it means that when you call a method that takes multiple seconds to complete (like refreshing your assetsà, the whole app will freeze for multiple seconds and you can't do any other operation in between.
I added another module rgb-lib/promises where all methods perform the tasks asynchronously instead and return a promise. It uses worker threads under the hood.

Usage:

const { Wallet, restoreKeys } = require("rgb-lib/promises");
const { WalletData, BitcoinNetwork } = require("rgb-lib");

async function main() {
  const { accountXpub } = await restoreKeys(BitcoinNetwork.Testnet, mnemonic);

  const wallet = await Wallet(new WalletData({
    pubkey: accountXpub,
    // ...
  }));

  const online = await wallet.goOnline(true, "ssl://electrum.iriswallet.com:50013");
  const balance = await wallet.getBtcBalance(online, true);
  
  // etc
}