feat: Add insta snapshot tests for formatter

[sorafujitani]

Summary

• Add snapshot tests for the formatter using Rust's insta crate.
• Make formatter output easy to inspect visually.
• Make formatting regressions easier to detect.

Test Cases

The snapshots cover formatter output for the following Ruby constructs:

• Class definitions, with and without inheritance
• Module definitions
• Method definitions, with and without parameters
• if conditional statements
• case/when expressions
• while loops

The tests parse Ruby source with the native Prism adapter and verify the output from the current Formatter pipeline.

How to Run

# Run the snapshot tests
cargo test snapshot_

# Review and accept updated snapshots
cargo insta review

How to Add New Tests

1. Add a test to ext/rfmt/tests/formatter_snapshots.rs.
2. Run cargo test to generate a new .snap.new file.
3. Review and accept the result with cargo insta review.

Closes #63