Use pull requests to trigger a new release

[joshmgross]

This splits the previous release workflow into two workflows:

1. Creates a release PR with a new package version
2. Creates the release and publishes the extension when the version is changed in package.json

This helps automate the release process and avoid having the workflows push directly to main (which doesn't work well with branch protection rules).

In the future, we can update the PR release workflow to include a changelog.