Add support for git flow using the git-flow-next tool (#5288)

This change enables Lazygit `git-flow` integration to work with
`git-flow-next`. The "official" `git-flow` has been deprecated and
replaced by `git-flow-next`. `got-flow-next` has the same tool and most
of the functionality is compatible but the `git config` is different so
Lazygit doesn't recognise it.

- Support gitflow.branch.<type>.prefix (git-flow-next) in addition to
gitflow.prefix.<type> (legacy)
- Refactor FinishCmdObj to use a prefix map lookup to find the branch
type