Split mnemonic in two parts using @iov/crypto

mnemonic_split.js

const original = 'dial ring shiver firm connect nominee stairs army large rubber deal crater'
const entropy = Bip39.decode(new EnglishMnemonic(original))

const part1Entropy = await Random.getBytes(entropy.length /* 16 bytes */)
const part2Entropy = entropy.map((byte, index) => byte ^ part1Entropy[index]) 

const part1 = Bip39.encode(part1Entropy).toString()
// 'pencil accuse tennis element peanut tent venue message process stove glory gym'

const part2 = Bip39.encode(part2Entropy).toString()
// 'panic roof boil axis mom enjoy change margin stove heavy forum food'