(Re?)introduce SASchema for .ZPM document type and update for "unwrapped" format

[isc-tleavitt]

Somewhat related to #501 - we need to reintroduce SASchema support and ideally expand to cover the "unwrapped" (e.g., top-level rather than {...} - bonus points if we can generate out SASchema for different resource processor types. A challenge may be differentiating attributes based on ProcessorClass - most relevant for non-built-in resource processors like from isc.ipm.js.

[isc-kiyer]

This is a good idea for the short term. For the longer term, would be great to support JSON where we can easily define a schema in the github repo and use $schema property to get intellisense which is better than the native XML autocomplete since it doesn't include descriptions of properties, attributes etc. which we can get with JSON.