Support multiple documents in yaml files

[nikolaik]

Quick hack to see how much work it would be ref #222

The output is a bit unintuitive atm, but could probably be improved:

$ cat catalog-info.yaml
---
apiVersion: backstage.io/v1alpha1
kind: System
metadata:
  name: example-system
spec:
  owner: my-team
---
apiVersion: backstage.io/v1alpha1
kind: Component
metadata:
  name: example-service
spec:
  owner:  my-team
  system: example-system

$ check-jsonschema --schemafile "https://json.schemastore.org/catalog-info.json" -v catalog-info.yaml
Schema validation errors were encountered.
  yolo.yaml::$: {'apiVersion': 'backstage.io/v1alpha1', 'kind': 'Component', 'metadata': {'name': 'example-service'}, 'spec': {'owner': 'my-team', 'system': 'example-system'}} is not valid under any of the given schemas
  Underlying errors caused this.
  Best Match:
    $.kind: 'Component' is not one of ['API']
  All Errors:
    $.kind: 'Component' is not one of ['API']
    $.spec: 'type' is a required property
    $.spec: 'lifecycle' is a required property
    $.spec: 'definition' is a required property
    $.spec: 'type' is a required property
    $.spec: 'lifecycle' is a required property
    $.kind: 'Component' is not one of ['Domain']
    $.kind: 'Component' is not one of ['Group']
    $.spec: 'type' is a required property
    $.spec: 'children' is a required property
    $.kind: 'Component' is not one of ['Location']
    $.kind: 'Component' is not one of ['Resource']
    $.spec: 'type' is a required property
    $.kind: 'Component' is not one of ['System']
    $.apiVersion: 'backstage.io/v1alpha1' is not one of ['backstage.io/v1beta2']
    $.kind: 'Component' is not one of ['Template']
    $.spec: 'type' is a required property
    $.spec: 'steps' is a required property
    $.kind: 'Component' is not one of ['User']
    $.spec: 'memberOf' is a required property