Add repository info for ablunit-test-runner extension

[kenherring]

• 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 adds the kherring.ablunit-test-runner extension so that it will be seeded to the Open VSX marketplace. I've had a few users which use IDEs other than VSCode make this request (see kenherring/ablunit-test-runner#421)