[ecash-lib] Patch import in mnemonic.ts

Summary: Here we imported from indexNodeJs.ts by accident, which breaks browser builds

Test Plan: npm test, also the bundle should now work in browser environments again...

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D17621