Add monaco-yaml for proper yaml support
Created by: eseliger
The basic yaml support built into monaco didn't suffice for the Batch Changes editor. The worker will only be loaded on that page, so this won't cause any additional payload to be loaded on any other pages. With this worker, we get
- Proper formatting
- Actual JSON schema support for auto completion, hovers and error detection
Stacked on top of https://github.com/sourcegraph/sourcegraph/pull/25886