Add ifsc() generator to en_IN bank provider

[Kunal8954]

What does this change

Adds an ifsc() method to the en_IN bank provider that generates
structurally valid IFSC codes (Indian Financial System Code).
Format: 4 letters (bank code) + 0 + 6 alphanumeric characters (branch code).

What was wrong

The en_IN bank provider only had bank() for generating bank names.
There was no way to generate IFSC codes, which are commonly needed
for Indian banking/fintech test data.

How this fixes it

Added a new ifsc() method following the same pattern as existing
locale-specific identifier methods (like pan()/gstin()). Added
a corresponding test in test_bank.py that validates the output format
using a regex pattern.

Closes #2435

AI Assistance Disclosure (REQUIRED)

• No AI tools were used in preparing this PR.
• If AI tools were used, I have disclosed which ones, and fully reviewed and verified their output.

Used Claude (Anthropic) for guidance on identifying the gap, writing the method, and drafting the test. All code was reviewed and verified by me (tests pass locally).

Checklist

• I have read the documentation about CONTRIBUTING
• I have read the documentation about Coding style
• I have run make lint

[Kunal8954]

"I've opened a PR for this: #2436"