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)?;

[caarloshenriq]

tACK

I tested the changes locally:

• Ran the full test suite (cargo test) and all tests passed.
• Performed manual tests by generating mnemonics with different WordCount variants (12, 18, and 24 words) using generate, generate_in, and generate_in_with.
• All generated mnemonics had the correct word count and the generation flow behaved as expected.

Everything worked as intended.

[tnull]

Thanks for looking into this! While it's a nice API improvement, it's probably not super critical so I think we want to hold off on merging this until we have another, more critical change that would require breaking API anyways.

In the meantime, here are a few comments. Please also always disclose in the PR description and commit message if you utilized AI tooling. To this end, rewriting the PR description and commit messages to replace AI slop with concise descriptions would be appreciated. Thanks!

[tnull]

Can we make this a impl From<WordCount> for usize?

[tnull]

I think we can remove this check now.

[tnull]

These changes here seem unrelated? Please drop them.