Remove rayon dependency and replace with std::thread-based parallelization

[sile]

This PR removes the rayon crate dependency and replaces its parallel iteration functionality with a custom implementation using Rust's standard library std::thread.

Key changes:

• Removed rayon and either from Cargo.toml and Cargo.lock
• Replaced rayon::iter::IntoParallelIterator with a custom filter_target_files function that uses std::thread::scope for manual thread pool management
• Added helper functions contains_stdin_path and should_run_in_parallel to handle parallel execution logic, including a warning when stdin is used with parallel mode
• Simplified parallel filtering in both format_files and check_files functions to use the new unified filter_target_files implementation
• Uses AtomicUsize and Mutex for thread-safe coordination without external dependencies