Add validation for readme.txt file and required header

[HarshithRajbangera]

What This PR Does

This PR adds a validation rule to the theme-review GitHub Action to ensure submitted WordPress themes include a valid readme.txt file with a required header.

Details:

• Verifies if readme.txt file exists
• Checks for the presence of === Theme Name === header inside the file
• Exits with an error if the file is missing or incorrectly formatted

Why It Matters

Most quality WordPress themes require a readme.txt with clear structure. This improves theme submission quality and saves reviewers time.

Type of Change

• ✅ New validation rule
• ✅ Improves theme QA automation

[HarshithRajbangera]

Hi team 👋

This PR adds a validation rule for ensuring that submitted themes include a properly formatted readme.txt with a required header.

I’d love feedback on any improvements or suggestions! I’m also actively looking to contribute more, so let me know where I can help.

Thanks!