feat: get `latestRelease` or `latestRelease.tag.version` equivalent in `PullRequestBody`

**Is your feature request related to a problem? Please describe.**
When generating a PR with Release Please, I would like to use information about the previous/latest release for:
1. Writing commit messages
2. Determining whether the update was a major, minor, or patch

**Describe the solution you'd like**
From my limited reading of the code, (please correct me if I'm wrong) all the data within `PullRequestBody` is available for the GitHub Actions script to use (outputs), which is what I will be using for simplicity. In this case, it would help me to know within the GitHub Actions script's outputs whether the release is a major, minor, or patch release, or I could do that on my own if I could get the latest release.

**Describe alternatives you've considered**
I could look for the tags within the repo manually and set a fallback if the repo is empty but since Release Please is already doing all this analysis, I think it could help to add this as a feature.

**Additional context**
I will primarily be using this to trigger updates from one Release Please repo in another Release Please repo. So essentially, all my repos will use Release Please and a major update in the parent will trigger a major update in the child, and so on, with minor and patches.

I'll be glad to submit a PR and work on this in whatever capacity required.