Add pan() and gstin() generators to en_IN provider

Hi! I'd like to work on this issue as one of my first open-source contributions. I'll add pan() and gstin() generator methods to the en_IN provider following the existing pattern (like aadhaar/ssn). Could you confirm this is still open for me to pick up?

Locate the en_IN provider and read its existing aadhaar and ssn generator methods to follow their pattern. Add the requested pan() and gstin() generators, then verify that the provider exposes both methods and that the relevant tests pass.

Written by the indexing model from the issue text.