Add ifsc() generator to en_IN bank provider

The en_IN bank provider currently only has bank(), which returns a bank name. It would be useful to also have an ifsc() method that generates a structurally valid IFSC code (Indian Financial System Code), used to identify bank branches in India — commonly needed for banking/fintech test data.

IFSC format: 4 letters (bank code) + 0 + 6 alphanumeric characters (branch code) Example: HDFC0001234

I'd like to work on this if maintainers are aligned.