Add dvc.yaml schema validation

[RandomFractals]

VSCode extensions can validate files with provided json schemas and use them for intellisense and code hints.

We should add dvcyaml-schema validation for dvc.yaml files, so users don't have to set it up manually after installing dvc and vscode-dvc extension.

[mattseddon]

I am unsure as to what this issue entails so I am closing for house keeping reasons.

[daavoo]

I think that including the dvcyaml-schema as part of the VSCode extension would be a nice feature to have. I regularly use VSCode for editing dvc.yaml and find the schema quite useful.

In addition, some ongoing initiatives on DVC (iterative/dvc#6446) might start to promote manually editing the dvc.yaml for "advanced" use cases.

[mattseddon]

No worries, I'll put this in tomorrow.