Add File Switcher extension

[lonix1]

• 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

"File Switcher" is a very useful extension which allows easy switching between source and test files.

A publishing request was made on the repo, but it's possible the process is too complicated, or there's no time for it.

Please auto-publish?

Thanks!