Support backport PRs in changelog-draft

`backport` creates cherry-pick PRs with a predictable format:

https://github.com/mapbox/github-release-tools/blob/20ca6c1ddfa321a2b3de9de81d18e30e316c3100/src/backport.js#L31

`changelog-draft` can use this to identify such PRs and use the original PR description instead.

cc @jfirebaugh

Start with src/backport.js at the linked line to understand the predictable cherry-pick PR format, then locate the changelog-draft entry point. Confirm that matching backport PRs use the original PR description instead, and verify the behavior with the project's available checks.

Written by the indexing model from the issue text.