Add support for GitHub Merge Queues

[mars-lan]

When using GitHub Merge Queue to merge a PR, the event name will be merge_group rather than pull_request.

[orgoro]

@mars-lan thanks! can you help verify that the event properties contain -

const {pull_request} = context.payload
const base = pull_request?.base.sha
const head = pull_request?.head.sha

base/head sha? if you can send a link to the object in the docs

[techman83]

There doesn't appear to be pull request details available in a merge group payload -> https://docs.github.com/en/webhooks/webhook-events-and-payloads#merge_group