GitHub actions for MLIR pull request proposal

I think for the MLIR pull request proposal we would need to implement our own action for auto-closing non-MLIR pull requests. The current solution we have for auto-closing does not do filtering.

I’ve been playing around with actions this week and it seems like it won’t be too hard too implement. I would also like to start using pull requests for stable releases soon, which would also need this smarter auto-close functionality.

Let me know if you need help!