Simplify Rust node generation with explicit naming (#51)

## Add explicit Rust names for AST nodes and simplify code generation

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