[concourse] Add support for set-pipeline step

[deepakmohanakrishnan07]

Concourse Language Server is used in multiple IDEs, SpringToolSuite, VS. Code, etc., to provide validation, content assist and documentation hovers for editing Concourse Pipeline and Task configuration files.

This PR includes changes to Concourse Language Server to provide the support for set-pipeline pipeline entity syntax highlighting, validation, and doc.

[kdvolder]

Thanks for the quality PR. Tests and everything :-)

[kdvolder]

Started getting some test failures after merging the PR.

https://ci.spring.io/teams/tools/pipelines/sts4-master/jobs/build-consourse-vsix-snapshot/builds/1666

For example:

[ERROR] Failures: 
[ERROR]   ConcourseEditorTest.primaryStepCompletions:299->assertContextualCompletions:5965->assertContextualCompletions:5969->assertContextualCompletions:5982 expected:<...ome-job
  plan:
  - []task: <*>
----------...> but was:<...ome-job
  plan:
  - [set_pipeline: <*>
-------------------
jobs:
- name: some-job
  plan:
  - ]task: <*>
----------...>
[ERROR]   ConcourseEditorTest.primaryStepHovers:363 Not found: Fetches a resource
 in 

[ERROR]   ConcourseEditorTest.relaxedIndentContextMoreSpaces3:4161 expected:<...in_parallel
- put
- []task
- try
- aggrega...> but was:<...in_parallel
- put
- [set_pipeline
- ]task
- try
- aggrega...>
[ERROR]   ConcourseEditorTest.setPipelineStepHovers:409 Not found: The path to the pipeline's configuration file.

I'm reverting the merge until I have to time to look into it in detail.