`check-gitlab-ci` fails to parse multi-document `.gitlab-ci.yml`

GitLab recently added the spec:inputs CI/CD YAML keyword which makes use of multi-document YAML files. The following example is taken straight from their docs:

spec:
  inputs:
    job-stage:
      default: test
    environment:
      default: production
---
scan-website:
  stage: $[[ inputs.job-stage ]]
  script: ./scan-website $[[ inputs.environment ]]

However, check-jsonschema refuses to parse such a file and errors out:

> check-jsonschema.exe --verbose --schemafile https://gitlab.com/gitlab-org/gitlab/-/raw/master/app/assets/javascripts/editor/schema/ci.json .\.gitlab-ci.yml
Several files failed to parse.
  FailedFileLoadError: Failed to parse .\.gitlab-ci.yml
    in "C:\Users\username\pipx\venvs\check-jsonschema\Lib\site-packages\check_jsonschema\instance_loader.py", line 50
    >>> data: t.Any = self._parsers.parse_data_with_path(

    caused by

    ComposerError: expected a single document in the stream
      in "<byte string>", line 2, column 1:
        spec:
        ^ (line: 2)
    but found another document
      in "<byte string>", line 10, column 1:
        ---
        ^ (line: 10)
      in "C:\Users\username\pipx\venvs\check-jsonschema\Lib\site-packages\check_jsonschema\parsers\__init__.py", line 102
      >>> return loadfunc(data)

Start by reproducing the failure with the multi-document .gitlab-ci.yml example and inspect instance_loader.py around line 50 and parsers/init.py around line 102. Update parsing behavior so this GitLab CI input is accepted, then verify that the check-jsonschema command validates the example without the single-document ComposerError.