GitHub action to block PRs into main

[flanakin]

⚠️ Problem

All PRs should be submitted against dev, but GitHub doesn't support that as a default PR branch when the code needs to show main for the latest stable release.

🛠️ Solution

Add a GitHub action that blocks PRs main and creates a new one against dev.

ℹ️ Additional context

We should also consider switching the default branch to dev and moving documentation out to a separate site. That would minimize the issue, but we still need to block PRs to main.

🙋‍♀️ Ask for the community

We could use your help:

1. Please vote this issue up (👍) to prioritize it.
2. Leave comments to help us solidify the vision.