Add chmod extension to extensions.json

[dlech]

I've had a user request to publish this package on the open exchange, so I would like that to happen automatically.

• 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

This PR introduces X change for Y reason.