feat: replace usize with WordCount enum for mnemonic generation

[coreyphillips]

This PR:

• Adds WordCount enum to restrict word counts to valid BIP39 values (12, 15, 18, 21 & 24)
• Updates generate(), generate_in(), and generate_in_with() to accept WordCount instead of usize
• Updates usage in comments and tests accordingly
• Closes Introduce enum WordCount #97
• ⚠️ This is a breaking change
  • generate() now requires use of the WordCount enum for the word_count parameter.
  • Migration:

    // Before
    let m = Mnemonic::generate(24)?;
    
    // After
    let m = Mnemonic::generate(WordCount::Words24)?;