Update extensions.json with the checkmarx repository

[phobot]

• I have read the note above about PRs contributing or fixing extensions
• I have tried reaching out to the extension maintainers about publishing this extension to Open VSX (if not, please create an issue in the extension's repo using this template).
• This extension has an OSI-approved OSS license (we don't accept proprietary extensions in this repository)

Description

Checkmarx One allows developers to access the full functionality of their Checkmarx One account directly from their IDE, eliminating the need for developers to use the CxOne platform. With this integration, you can initiate new scans, review scan results, and receive guided remediation advice. Checkmarx offers comprehensive details about each vulnerability, including remediation recommendations, examples of effective fixes, and AI-generated code suggestions. The extension also lets you quickly navigate from a vulnerability to the associated source code, making it easier to identify and address problematic areas

This PR introduces Checkmarx extension change because I like to use it on my machines.