`dancelist validate` should support yaml anchors

[torotil]

I’ve just tried to create a PR for the Vienna events (#447). According to my local schema validations the yaml file is fine, but it fails to validate with dancelist validate because I’ve used anchors to spare me some copy&paste.

I’m not a rust programmer but according to https://stackoverflow.com/questions/73311581/how-to-use-yaml-references-with-rusts-serde-yaml serde_yaml (which seems to be used by this project) supports anchors. Therefore I’m not sure what’s wrong with dancelist validate.

[qwandor]

This might be the issue from dtolnay/serde-yaml#317, which also mentions a workaround. I'll try that when I have time.