fix: add missing file extension to import

[Azlond]

The type import of PostPolicyResult in src/internal/client.ts is missing its file extension.

This can cause issues with projects requiring file extensions on import paths:

node_modules/minio/dist/esm/internal/client.d.mts:11:39 - error TS2307: Cannot find module '../minio' or its corresponding type declarations.

11 import type { PostPolicyResult } from '../minio';
                                         ~~~~~~~~~~


Found 1 error in node_modules/minio/dist/esm/internal/client.d.mts:11

I've added a .ts file extension similar to the import of xml-parser.ts, which also only exists as .js-file