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.

Locate the en_IN bank provider and its existing bank() implementation, then inspect nearby provider tests for the expected generator patterns. Add ifsc() so its output follows the stated 4-1-6 IFSC structure, and run the relevant provider tests with coverage for the format.

Written by the indexing model from the issue text.