Describe the feature or problem you’d like to solve

markProjectV2AsTemplate and unmarkProjectV2AsTemplate mutations exist. Inputs in shurcool/githubv4 do too, for example https://github.com/shurcooL/githubv4/blob/599ae7bbf278c79d7354974febd1f6684dfe30cd/input.go#L1809

Let's add new commands:

• Mark as template gh project template 1 --owner "github"
• Unmark as template gh project template 1 --owner "github" --undo

And add the template field to the project JSON.