Add support for creating bundles

[andli197]

It would be useful to add support for git bundles in magit. From what I can see there is no implementation supporting this and I always find myself needing to use a shell for this task instead.

[tarsius]

This would be a good project for someone who wants to learn how to use transient.

[kwas]

I'd like to work on this issue

[tarsius]

Great!

Let me know if you have questions. My first tips would be to use a separate file and learn from other, fairly self-contained menus that are part of Magit.