Refactor NIF to idiomatic Rust

[alchezar]

Summary

Refactored the HTML to text conversion NIF to follow idiomatic Rust patterns and improve code maintainability.

Changes

• Static atoms — replaced runtime Atom::from_str() calls with rustler::atoms! macro for better performance
• Options struct — extracted configuration into a dedicated struct
• Trait implementations — added TryFrom<Term> for parsing and From<Options> for conversion to html2text config
• Macro for repetitive code — introduced set_if_ok! macro to reduce boilerplate in option parsing
• Documentation — added doc comments for items

Benefits

• Cleaner separation of concerns
• Better readability and maintainability
• Slightly improved performance (static atoms)
• No functional changes — existing API remains the same