Add `merge_group` build trigger

[yeikel]

What problem are you trying to solve?

Add the merge_group trigger to a build so that builds are triggered in the merge queue

What precondition(s) should be checked before applying this recipe?

This depends on enablement of merge queue in the branch protection rules but it is out of the scope of this project

Describe the situation before applying the recipe

on:
  pull_request:

Describe the situation after applying the recipe

on:
  pull_request:
  merge_group:

Any additional context

The generic recipe #90 could be used to accomplish this but as that recipe will be more involved, we can deliver this independently first.

https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/configuring-pull-request-merges/managing-a-merge-queue

[yeikel]

@timtebeek What are your thoughts about delivering this before #90?

[timtebeek]

Should be manageable I think; at first glance it's no more than providing a named recipe that applies MergeYaml to add in that merge_group in $.on, very similar to what we've done here:

rewrite-github-actions/src/main/resources/META-INF/rewrite/github.yml

Lines 17 to 29 in 5c36536

	type: specs.openrewrite.org/v1beta/recipe
	name: org.openrewrite.github.AddManualTrigger
	displayName: Add manual workflow trigger
	description: You can manually trigger workflow runs. To trigger specific workflows in a repository, use the `workflow_dispatch` event.
	tags:
	- github
	recipeList:
	- org.openrewrite.yaml.MergeYaml:
	key: $.on
	yaml: |-
	workflow_dispatch:
	acceptTheirs: true
	fileMatcher: '.github/workflows/*.yml'

Welcome to push that up! 🙏🏻