Fix index.ts regeneration on traits changes

[skryukov]

This pull request addresses a bug in the code generation process, ensuring that the index.ts file is correctly regenerated when traits are added or removed. The main change updates how the index.ts file is written, which improves the reliability of the generated TypeScript interfaces.

Bug fix for code generation:

• Updated write_index in lib/typelizer/writer.rb to use the fingerprint of interfaces instead of their filename when generating index.ts, ensuring the file is regenerated when traits change.
• Documented the fix in CHANGELOG.md under the "Fixed" section, describing the issue and its resolution.
• Updated the snapshot digest in spec/__snapshots__/index.ts.snap to reflect the new output from the improved generation process.