BIP39 passphrase support

[beemeeupnow]

HDNode.fromMnemonic accepts such a password, but it does not get stored and Wallet just passes null when checking mnemonic/address matching.

Ref:

ethers.js/packages/wallet/src.ts/index.ts

Line 64 in 7b134bd

const node = HDNode.fromMnemonic(mnemonic.phrase, null, mnemonic.locale).derivePath(mnemonic.path);

This causes problems if I use HDNode.fromMnemonic and pass a password, then later use .derivePath() and pass that HDNode to create a new Wallet (result is failing the check right after the above line)

It seems trivial to fix, but I wanted to check what would be recommended. (happy to send a PR)

e.g. could add the passphrase to HDnode directly and store it so it can be retrieved later and passed during the above call in the code ref, or could add it to the Mnemonic interface and retrieve it from that

[beemeeupnow]

Reading BIP39 it makes sense to add to Mnemonic because it is used in combination with the phrase.

As I need this feature I opened PR #2721