refactor: modularize rust bindings following Ruby's structure

Reorganize ruby-prism crate to mirror lib/prism/ structure:

- node.rs: NodeList, ConstantList, Integer (like node.rb)
- parse_result/mod.rs: ParseResult, Location (like parse_result.rb)
- parse_result/comments.rs: Comment, MagicComment (like parse_result/comments.rb)
- parse_result/diagnostics.rs: Diagnostic (like parse_result/errors.rb)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: sei40kr