feat(process): Unwrap List types

[sh0rez]

This PR adds native support to Tanka for unwrapping List objects, those
being k8s pseudo objects used to bundle multiple resources into one
object.

To detect whether something is a List or not, the existence of items
is used, the same criteria the offical k8s.io packages use:

// https://github.com/kubernetes/apimachinery/blob/61490fe38e784592212b24b9878306b09be45ab0/pkg/apis/meta/v1/unstructured/unstructured.go#L54-L61
func (obj *Unstructured) IsList() bool {
	field, ok := obj.Object["items"]
	if !ok {
		return false
	}
	_, ok = field.([]interface{})
	return ok
}

Also refactors SchemaError which is now used in Unwrap as well. The error now also displays a code snippet, so debugging becomes easier

Fixes #277

[malcolmholmes]

Tried this out.

Added this out first:

  list: {
    kind: 'List',
    apiVersion: 'v1',
    items: [{
      kind: 'Deployment',
      apiVersion: 'apps/v1',
    }],
  },

It complained:

.list.items[0] has missing or invalid fields: metadata, metadata.name:

  apiVersion: apps/v1
  kind: Deployment

Please check above object.

This complaint seems completely reasonable. I followed its instructions:

  list: {
    kind: 'List',
    apiVersion: 'v1',
    items: [{
      kind: 'Deployment',
      apiVersion: 'apps/v1',
      metadata: {
        name: 'foo',
      },
    }],
  },

This rendered as expected, the deployment was in the list of resources, and there was no list in the output.

[mplzik]

LGTM, but note that this also does refactoring of SchemaError that is not mentioned in the description; might be worth adding it to the description.