Implement a warning system for data model validations

[pimeys]

Now we only warn through the language server. We should have a system in place, that would output warnings in a similar way as we do with errors. Yellow instead of red highlight, and not stopping the execution when having only warnings.

Open questions:

• When to warn, always when evaluating a data model
• Do we warn about things such as deprecated features in the cli additionally
• Is it ok to be noisy?
• And if we're not noisy, can we reach the users with deprecation warnings