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'
