spec: Add an optional $schema property at top level

[dolmen]

At top level, add a $schema property to allow to refer to a JSON Schema.

[nichtich]

This only makes sense if there can be multiple schemas and schema locations, depending on extensions, application profiles etc.

[dolmen]

This also helps to document that a JSON Schema exists to validate the document. A text editor might use that to automatically enable linting.

[abitwired]

I think it makes sense to define a $schema for the node objects. Most applications will want more complexity than a single text attribute on a node. Nodes often represent services or lambdas. Applications using the node API should be allowed to shove their own metadata inside the node for the data relevant to their application's purposes.

e.g., an application which uses the infinite canvas to manage an ETL process. Such an application would need more than a text node. They would likely need additional attributes for each node to help define each ETL process. With the existing API, there is no way to include those additional attributes.

The $schema attribute should not be required for import/export of .canvas files since that would hurt interoperability between projects. They should still be able to load the graph relationship of the nodes/edges without necessarily needing to understand the custom $schema used by a different application.

Is what I described within the scope of your suggestion @dolmen ?