support BIP39 mnemonic in descriptors

Supporting BIP39 mnemonic in descriptors would allow less painful imports and scans of third party wallet seeds.

I think it could be used almost identical as the existing xpriv key.

Example: pkh(bip39(room,cross,cube,glance,infant,setup,renew,more,lion,glimpse,retreat,tone,chief,vanish,brisk,destroy,destroy,bounce,knee,analyst,autumn,meadow,divorce,since)/44'/0'/0'/0/0) (or slightly different to support the "passphrase")