Append new content to `.gitignore` or `deno.json` if it already exists

[ooker777]

In the case .gitignore or deno.json already exists, it's more convenient if the command appends data to it rather than overwrite it. If the user already uses deno.jsonc, it won't create a new deno.json

[oscarotero]

That's a good idea. I'll work on that.
thanks!