Allow users to create new commits

[icorbrey]

Description

I want a way to create a new commit from within jjj. Ideally, I should be able to e.g. press <n> while a single commit is highlighted to run jj new <change ID> on that commit.

Use Case

Core Jujutsu functionality.

Last Steps

• I agree to abide by this project's code of conduct.
• I would like to work on this and submit a PR with my changes.
  (Optional, but contributions are greatly appreciated!)