[llvm] [GitHub][workflows] Ask reviewers to merge new contributor's PRs (PR #80659)

[David Spickett via llvm-commits]

DavidSpickett wrote:

This workflow will for certain miss people who are not new but don't have commit access, I'm aware of that limitation.

My intent for now is to get the greeting - merge on behalf - post merge comments working for new PRs. Then after that look into something more precise like Graph QL queries to apply these sorts of workflows to any author.

https://github.com/llvm/llvm-project/pull/80659