feat: Add workflow_run.workflows references check

[gmazzo]

Implements #83 by introducing a new workflow_run rule to check the existence of referenced workflows at that trigger.

This rule is a bit different than others, it will be a singleton instance for the check run (instead of one per file), to allow collecting workflow names from the whole suite.

Because of that, it can only apply when calling LintFiles (nil for other usages of the linter).
It validates that any name reference on workflow_run.workflow exists in another workflow of the same set.

on:
  workflow_run:
    types: [completed]
    workflows:
      - build
      - test # ERROR: This workflow does not exist, its name is `Tests`

This is my first experience working with Go, feel free to suggest any change regarding code style or best practices.

[gmazzo]

I need the path here to infer the name from the filename, because it's not always defined explicitly on the workflow's YAML.

I also explored adding a new VisitFile method to avoid touching too many files, but because of the stateful nature of this rule, and the concurrent approach for processing files, the current name stored as a field of the rule itself gets overridden, leading to a race condition.

So I end up adding this extra parameter here to compute the final name.

I could also add the final name to the Workflow structure, but it didn't feel right, as that stands for a pure YAML model.

[gmazzo]

Hi @rhysd, I see there is activity in this repo. Any chance to get this one reviewed? We are currently using a forked internal version of your tool.

Great work by the way!