Describe the feature or problem you’d like to solve

We're able to change the base branch of a PR with the web UI, and the API, just not with gh. Would like to add a --base flag to gh pr edit

Proposed solution

Add --base flag to gh pr edit, which uses the base field on the PATCH call for the PR

Additional context

https://docs.github.com/en/rest/reference/pulls#update-a-pull-request documents the base field on the PR update call