feat(keyring-sdk): add decodeMnemonic{,Words}

[ccharly]

Decoding helpers for mnemonics.

---

Note

Low Risk
Additive SDK API with tests and no changes to existing encode behavior; mnemonic handling is sensitive in principle but this only adds parsing helpers consistent with existing encoders.

Overview
Adds decode helpers in keyring-sdk that mirror the existing encodeMnemonic / encodeMnemonicWords APIs, turning BIP-39 English mnemonics back into 16-bit word-index byte buffers.

decodeMnemonicWords splits a space-separated phrase and maps each word through the English wordlist; decodeMnemonic UTF-8-decodes bytes first, then delegates to decodeMnemonicWords. Both reject unknown words with Invalid mnemonic word: "…". Tests cover empty input, round-trip with encodeMnemonic, and invalid-word errors. The unreleased changelog entry documents the new exports.

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[ccharly]

@metamaskbot publish-preview

[github-actions]

Preview builds have been published. See these instructions (from the core monorepo) for more information about preview builds.

Expand for full list of packages and versions.

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