feat(node): add typings

[mittalabhas1]

Adds .d.ts type declaration files for the Node.js package so TypeScript users get type checking and IDE autocompletion out of the box after installing via npm/yarn — no separate @types/ package needed.

Changes

• src/node/index.d.ts — types for validate, fetchDetails, and bank; includes IFSCDetails interface matching the API response shape
• src/node/bank.d.ts — types for the generated bank constants object
• package.json — adds "types" field pointing to index.d.ts

Notes

Uses the export = + namespace pattern so it works regardless of whether the consumer has esModuleInterop enabled in their tsconfig.json.

No runtime changes. Existing tests pass.

[mittalabhas1]

@PriyankaMarbill whenever you get a chance ^