Add brittanychiang.halcyon-vscode

[bchiang7]

• 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 adds the Halcyon Theme extension change from VSCode.

https://marketplace.visualstudio.com/items?itemName=brittanychiang.halcyon-vscode
https://github.com/bchiang7/halcyon-vscode