build: include bundled TypeScript declarations

[jeanpierrecarvalho]

Summary

• add bundled index.d.ts copied from the current @types/cors definitions
• expose the declarations through the package types field and include them in the published files list
• keep the legacy headers option alias typed, marked as deprecated, to match current runtime behavior

Why

Issue #235 asks to ship TypeScript definitions with cors instead of requiring a separate @types/cors install.

This change keeps the types lightweight and aligned with the current middleware API while improving the out-of-the-box TypeScript experience for consumers.

Impact

TypeScript users can install cors alone and get bundled declarations immediately.

Existing code using the legacy headers alias continues to type-check, while new code is steered toward allowedHeaders.

Validation

• npm test
• local Express + TypeScript smoke test against the file-linked package, including cors(), cors.CorsOptionsDelegate<Request>, and the legacy headers alias

Addresses #235.