Add Custom Region Support to Yaml Files

Is your enhancement related to a problem? Please describe.

At times there are areas of a yaml file (like a block of anchor definitions) that take up a large portion of the file, and while they may not be under a top level key, a user would like to fold them so as to more easily read and navigate the yaml file.

Describe the solution you would like

Currently, with JS in vscode, you can add a comment such as // #region and // #endregion to support this. Currently yaml in vscode does not support this type of functionality, and that comment key is not valid in yaml.

Something like # !region or the like so we could have this functionality in a yaml file would be really nice

Describe alternatives you have considered

There aren't really any alternatives to this, as some things you'd like a custom region for are top down, and as such, you can't put at say, the bottom of the file.

The issue does not name files, tests, or an entry point. Start by locating the repository's existing YAML folding behavior and compare it with VS Code's region support; agree on the marker syntax and verify folding for top-level anchor blocks and other YAML sections.