Simplify Rust node generation with explicit naming

Instead of auto-generating nested module paths from RBS nested naming
conventions, use explicit rust_name fields in config.yml and generate
flat structs.

- Add rust_name field to all node definitions in config.yml
- Remove complex module/path parsing logic from build.rs
- Generate flat structs (e.g., ClassNode) instead of nested modules
- Add Node enum to wrap all node types

This makes the generated Rust code easier to work with.

Author: alexcrocha