The validator should return `warnings` as a result from the schema validation

[marcellobarile]

Is your feature request related to a problem? Please describe.

At the moment, when, for example, a template is edited via an editor (e.g. Monaco), warnings are shown inline:

but not returned by the validator:

Although we could argue that those are not blocking, it would still be useful to have them returned so that consumers can enrich their UI without relying on another editor.

Describe the solution you'd like

I would expect the validator to return not only errors but also warnings.

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Related to camunda/element-templates-json-schema#196 (comment)

[nikku]

We had related discussions in the context of implementing the first deprecated properties, cf. camunda/element-templates-json-schema#196 (comment).

From the API perspective, how would you assume that would work? This is the main complication that I think we'd need to overcome. For reference, this is how bpmn-js solves it:

const {
  warnings
} = await viewer.importXML('SOME_XML');

[marcellobarile]

For the sake of consistency with existing bpmn-io APIs (like the one mentioned above), I would expect the same data structure; alternatively (if this is considered a breaking change), I would expect a level property to be exposed for each problem returned by the validator.