feat(keyring-sdk): add encodeMnemonic

[ccharly]

Adding a small helper to encode from 16-bits indices to UTF-8 mnemonic string.

This will mostly be used by @metamask/eth-money-keyring integration.

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Note

Low Risk
Low risk: adds a small, well-tested utility and a new dependency; main risk is incorrect wordlist/index handling or unexpected bundle/dependency impact.

Overview
Adds a new public helper, encodeMnemonic, to @metamask/keyring-sdk that converts a Uint8Array of 16-bit BIP-39 word indices into UTF-8 bytes for the corresponding English mnemonic phrase.

Exports the new module from src/index.ts, adds unit coverage for empty/single/multi-word cases, and introduces the @metamask/scure-bip39 dependency (with changelog and lockfile updates).

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