chore(README.md): fix formatting and add OpenVSX badge

[matifali]

Also Closes #269

[Copilot]

Pull Request Overview

This PR fixes formatting issues in the README and adds an Open VSX badge to promote the extension on the Open VSX registry.

• Adds an Open VSX badge to increase visibility.
• Updates code block language from text to shell for better syntax highlighting.
• Adjusts markdown formatting for headings and inline code in the Variables Reference section.

Files not reviewed (1)

• .prettierrc: Language not supported

Comments suppressed due to low confidence (2)

README.md:37

• [nitpick] Ensure that changing the heading from '#### Variables Reference' to '### Variables Reference' maintains consistent structural hierarchy across the document.

### Variables Reference

README.md:30

• [nitpick] Confirm that '```shell' is the most appropriate language identifier for the command; if the command benefits from a different identifier, consider updating accordingly.

```shell

[EdwardAngert]

👍