Add var_sources to Concourse schema

[ThibaultDelaune-pro]

Describe the bug

Concourse CI Pipeline Editor VSCode extensions are falsely reporting a problem with var_sources.

Error message:

Unknown property 'var_sources' for type 'Pipeline' vscode-concourse(YamlSchemaProblem)

This request is really similar to #942, maybe you could combine them...

To Reproduce

---
var_sources:
- name: global_vars
  type: dummy
  config:
    vars:
      pipeline-path: pipelines/example.yml

resources:
  - name: concourse
    type: git
    icon: github
    source:
      uri: https://github.com/concourse/concourse.git
      branch: main

jobs:
  - name: example
    plan:
      - get: concourse
      - set_pipeline: examples
        file: concourse/((global_vars:pipeline-path))

Fix

Please add var_sources to Concourse schema.

cf. https://concourse-ci.org/vars.html#var-sources

[BoykoAlex]

Fixed with 1812584