support merge_method options

[robandpdx]

Merges will fail if the repo is configured to disallow merge commits for PRs. Add an option to configure the merge_method to be used for merging.

https://docs.github.com/en/rest/pulls/pulls#merge-a-pull-request

Values can be merge,squash, or rebase. Default to merge.

We can check the pull request.labeled event to see what merge method is allowed. If merge commits are allowed, use merge, else try squash and finally use rebase as a last resort.

if (event.pull_request.base.repo.allow_merge_commit == 'true') {
  mergeMethod = 'merge'
} else if (event.pull_request.base.repo.allow_squash_merge == 'true) {
  mergeMethod = 'squash'
} else {
  mergeMethod = 'rebase'
}