audit: detect repeated CSS rules (fixes #20)

[Shekhar0109]

Description

This PR adds a new audit to detect repeated CSS rules, as described in Issue #20.

Changes Made

• Added audits/repeated-rules.js to check for duplicate property:value pairs across selectors.
• Integrated the audit into the main runner (index.js).
• Provided warnings with selector + property + value when duplicates are found.
• Updated README.md to include the --repeated-rules audit and instructions on how to run it.

Why

Repeated CSS rules increase file size and can indicate poor maintainability.
This audit helps highlight them automatically and ensures the documentation reflects this new feature.

Testing Instructions

1. Add CSS with duplicate rules, for example:

   .foo { border: 1px solid red; }
   .bar { border: 1px solid red; }

2.Run the audit tool:
npm run css-audit -- path/to/test.css --repeated-rules
3.Verify that duplicates are reported correctly.

##Checklist
[x]I have tested with a sample CSS file.
[x]Code follows project conventions.
[x]README updated with --repeated-rules audit instructions.

Fixes #20