`gs branch checkout`: Ability to track untracked branches by default

[injust]

I have the opposite use case of what https://abhinav.github.io/git-spice/cli/config/#spicebranchcheckouttrackuntrackedprompt solves: I frequently checkout untracked branches that I want to track, but I find the prompt repetitive.

The spice.branchCheckout.trackUntrackedPrompt option also confused me because I thought setting it to false would suppress the prompt while keeping the behaviour that the prompt defaulted to (which is to track the untracked branch). Instead, the prompt is suppressed and the branch is left untracked.

[abhinav]

This sounds reasonable. An option for branch checkout to auto-track branches that aren't tracked.