We were formally targeting ES2015 and relying on TypeScript desugaring
the use of newer features like async/await, which is unnecessary.
Desktop targets ES2020; we can too. One thing TypeScript was *not*
desugaring was BigInt literals, which we can now use freely.
Previously all APIs were available through the top-level index.ts, but
now the zkgroup APIs are in their own module. To access that in the
old packaging required writing
import * from '@signalapp/signal-client/node/dist/zkgroup'
This commit moves the package root into the node/ directory to
eliminate the 'node/' component, then adds a top-level, precompiled
zkgroup.js/.d.ts so that clients can use
import * from '@signalapp/signal-client/zkgroup'
