[Cashtab] [BCH Deprecation] [Mnemonic] Pt 6/7 - Optimize bip39 bundle via webpack IgnorePlugin

Summary:
T2730

Depends on D12482

Stacked diff to localize existing uses of BCH.Mnemonic throughout the app and deprecate the BCH.Mnemonic object.

This diff uses webpack.IgnorePlugin to create a bip39 bundle that would only contain the english wordlists.

Pt 1/7 - Implement BCH.Mnemonic.toSeed locally
Pt 2/7 - Implement BCH.Mnemonic.generate locally
Pt 3/7 - Implement BCH.Mnemonic.wordLists locally
Pt 4/7 - Implement BCH.Mnemonic.validate locally
Pt 5/7 - Deprecate BCH.Mnemonic
Pt 6/7 - Optimize bip39 bundle via webpack IgnorePlugin
Pt 7/7 - Unwrap validateMnemonicWordList

Test Plan:

• npm test
• npm start
• insert a valid mnemonic into the Import Wallet field and ensure the english wordlist is displayed along with a 'bip39 wordlist is excluding japanese, spanish, italian, french, korean, czech, portuguese and chinese' console log message
• comment out the webpack.IgnorePlugin config in webpack.config.js and observe that the console log message above for excluding non-english wordlists is not displayed

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D12526